Entity attachments are extremely common. We all have them unless we clear ourselves out several times a day - depending on what type of environment we are moving through in our daily routine and how much psychic pollution surrounds us. The strength of our auric field also determines the amount and degree of attachments we pick up. These days, with all the electromagnetic pollution and interpersonal crud we move through, it's best to clear yourself out several times a day anyway!
We're also born with a lot of core attachments, so there is a lot of gunk we come in with. And some of it is tied to actions we've taken, illnesses we have experienced, and the death injuries we experienced in past lives. Other stuff we pick up from our physical parents, even the hospital in which our mum was treated when she gave birth to us; other stuff is genetically based across many generations and has an ancestral, tribal origin.
So the whole process of doing things like taking a nice soak in a bathtub with some sea salt at least once a week becomes really important - to learn to consistently clear off both the electromagnetic pollution we're constantly picking up from computers, cell phones, TV's, etc and also to clear out lower level elemental "cling-ons" plus discarnates, ET's, and yes, demons. Salt tubs rock! If you don't have a bathtub, you can use a bottle of water with some sea salt in it and spray sea salted water throughout your energy field. You'll notice a big difference in how you feel. The salt has the ability to clear out both electromagnetic gunk and parasitic etheric entities. It's one of the easiest, fastest things you can do to clear out. Add a little baking soda, too. I like to use about a quarter cup of salt and a few shakes of baking soda.
Artichoke, if your boss went through treatment at a hospital, there are loads of entities at hospitals hopping onto patients. Just about the only way to prevent this is to a) invoke for clearing of the hospital before you set foot in it; b) invoke for clearing of the staff - doctors, nurses, tech help and c) set up a field/shield of protection around yourself using the colors, frequencies, symbols, or whatever works for you to block this stuff out. Then you can pass in and out of the hospital entity-free.
You can do this on behalf of another person once you feel comfortable with the steps in the process, and it works just fine to hasten the person's recovery, manifest the best possible conditions for their treatment, etc. I also like to leave a permanent invocation in place for angelic and spiritual helpers to continually keep the place clean for as long as it is used as a hospital. There are hosts of higher beings who are happy to help with this work.
(Sidenote - the only hospital I've been in that had very few entities "stuck" there was a Catholic hospital where Mass was conducted throughout the day. All that prayer really worked to clear things out! It was pretty remarkable, the difference in that place vs. a non-religious hospital.)
When you go under anasthesia (and didn't your boss go under during an operation?) you have to have invoked MAJOR PROTECTION to watch over both your physical body (so no uninvited guests enter) and also you need to invoke for major protection around your etheric body, which will tend to stroll around in a somewhat discombobulated way because of the effects of anasthesia and can be waylaid by negative entities who love to feed in hospital settings. It's possible your boss did come home with a "hijacker" who has assumed control of the helm.
Drugs of any kind, especially the strong beta blocker family of drugs, can also whack out the etheric and physical bodies and create holes, cracks, fissues in the energy field - again, when the "egg" or the auric field becomes weakened, opportunistic critters come in.
I know it sounds complicated. But it doesn't have to be. You can approach this with an attitude of humor, clarity, and fun and master it over time. The more you focus on spiritual sovereignty, power prayer (the kind of invoking of the Divine that carries NO doubt, absolute clarity and confidence,) and metaphysical wisdom, the better you get at it all.
Learning to clear out the aura and strengthen it also REALLY helps your physical health and physical immunity. I haven't been sick in the four or more years since I finally starting getting to grips with this stuff. This, when others around me have been stricken by bad flu, bronchitis, you name it.
The unensouled being thing is interesting.....I can say that many people I've met are partially possessed, and that a smaller percentage of them are VERY possessed to the point of not being human anymore. But very few were unensouled.
To clarify - I'm definining "possession" as having other entities overlaying your core, essential being, and these overlaying entities gradually fool you over time into believing that the core YOU has certain flavors, emotional patterns, and attitudes.....which after you are cleared out, you'll realize ARE NOT YOU AT ALL. Huge, bizarre fetishes for certain things, or addictions to things, are often gone from a person after they have removed these entity layers. You can go your whole life thinking, say, that steam trains are the cat's meow and then, after you remove entity attachments, you realize that that passion for trains was an overlay from a discarnate attachment imposing his tastes and addictions onto you. And from then on, you don't give a piss about trains anymore! Very interesting how what we think of as our core "personality" can often just be an overlay of critters imposing their tastes and addictions onto us.
Things get very ....quiet......and peaceful....after entity attachments are cleared out of you. And when things get quiet, you can move through life as a content observer, disengaged from the push/pull of the normal human emotional drama, and just enjoy the ride.
A possessing being may be a discarnate, even someone who died and still loves you. Helpful Heloise tip: Stop praying to your dead Daddy to be your spiritual guide! He has his own business on the Other Side to attend to! (I had a friend who would not listen to me about this one - sure enough, Daddy is now stuck in her energy field and preventing his daughter from ever getting a date again - because nobody is good enough for his little girl and he energetically pushes all her potential suitors away. UGH.)
Or a possessing being can be a complete "stranger" discarnate who has gained entrance. (It can also be more demonic ghoul types of energy or a mixture of all of them glommed together.)
Discarnate attachments are one of the primary causes of schizoprehnia. I've treated and cured people with schizophrenia and it was all due to entity overlays and attachments. But if the person didn't take steps to do their OWN homework, reclaiming spiritual sovereignty and avoiding drugs, actions, and relationships that weakened their auric field, the voices eventually came back. The one guy complained that he "missed" the voices after his lifelong schizophrenia was cured....sure enough, in a few months, they were back. Who wouldn't come back, after an invitation like that!
Partial possession is different than a total absence of soul, though. The average guy or gal has a few layers of gunk in their field; but some folks have this gunk almost down to their spiritual core. They are the deep-seated possessed ones.
The deeply possessed ones haven't a clue about it, either. They're usually the ragers, the depressives, the drug addicts, or the types who have the most intensely miserable lives with lots of emotional drama that is constantly being acted out. They become psychic vampires constantly seeking "loosh" from others and have lost access to any internal light, the self-sustaining kind, within them. These types can NOT be helped unless they start to become more self-aware. They plague the doorsteps of healers and therapists and become a royal pain in the ass because professionally, healers have pledged to help everybody. So how do they justify turning a patient away because, frankly, they have lost touch with the immortal power of their soul and are simply leeching energy from everybody in their environment?
I eventually learned to turn those people away in my full-time counselling practice. But I was already severely ill and burned out from having naively tried to help these people for over a decade. Then I spent years clearing myself out and curing myself through shamanic healing, pranic healing, and entity release therapy. ( I'm all better now, thank you 
We have so many layers of entity attachments from past lifetimes that the main trick I've found is to RENOUNCE ALL TIES OF ALLEGIANCE to any gods/goddesses (hint: ET's and demons) we've pledged service to throughout all lifetimes and realities. This can REALLY help you on your way to clearing core stuff out.
Then you reorient yourself by stating clearly, passionately, and repeatedly, "I am a spiritually sovereign being; I reclaim my spiritual sovereignty" until the ghouls start to leave you alone.....and this new clarity helps you to instantly spot when a critter is trying to trip you up and enter your life, your energy field, or your partner's energy field. And then you can kick them out from a place of strength. You'll also find yourself doing this in lucid dreams, clearing out critters seeking entrance through the dream realm.
Most of us are too weakened by deep-seated entity attachments to be of much use to others as healers until we ourselves have cleared out a great deal of goop. I hate to say it, but pretty much 99% of all psychics I've known had a lot of possession going on...mainly because they're so into worshipping this goddess or that god, etc. ad nauseaum, and Gods Dammit, they are all freakin' ET'S! Get a grip, people! (Thanks for letting me vent. I feel better now.)
I have met some unensouled - what do you call them? people? they're not people. Humanoid, perhaps, certainly with a human-seeming physical body - but that could simply be a holographic projection, so who really knows WHAT they are? These beings do not have a soul - you can sense the complete absence of it. Quite an eerie thing. It's much more eerie than encountering a human whose heart chakra and upper energy centers aren't open - those people aren't my favorites because they are so dark and heavy in vibration, but at least they still have a soul in their body!
The unsouled are completely Luciferian, they are from the void, and they are stealing more and more bodies here.
But then, on the opposite side of the spectrum, there are vast spiritual armies quietly going about the business of kicking the Luciferians' butts. And angels walking around in human "clothing" too. Well, they use holographic bodies temporarily to effect a positive change in this realm. Angels don't actually incarnate, because angels are bound by universal law not to enter human bodies. I wish the channelers would understand that one. Angels can communicate, hover in a person's energy field, and communicate holographically from soul to soul. But they do NOT take over a voice. They do NOT take over a body. That's how you can distinguish the real ones from the Luciferian pretenders.
Okay, this is my mad ramble for the day. I get excited talking about entity attachment stuff since I've spent so many years working with this form of healing, but it's a little like having a dirty little hobby nobody wants to hear about.....so I usually just keep my mouth shut about it. It's nice to have a place to rant!
Lipstick Mystic aka Jennifer
Being that you're never going to get a clear cut answer on the issue, there's probably no sense in going in circles. Also, if you're going to theorize that he's been replaced, you'd have to then ask yourself why. Why would someone do that? What's the motive or agenda? What purpose would he be serving as a reanimated robotic agent? To which you'll only have theories, and never any definitive answers. 
It was a very informative and useful post. Entity attachments is a huge and important subject. If anybody's interest is piqued after reading your post then a good recommendation is Robert Bruce's "Practical Psychic Self Defense." VERY useful handy book, and practical too, just like the title says!