Little Details

Someone with long-term PTSD who has also been through physical isolation (not to this severe degree! But to a degree where I see psychologists who have had to research this stuff, and I've had to research it too.)

You may find that the things that disturb his life most are actually around the trauma of being forced into bare bones survival vs. contemplating the malicious engineering etc.

* He'll likely have big issues with food hoarding, including putting food into pockets for later, food storing, buying in bulk, and signs of significant eating disorders that could go in any direction, from anorexia (control + 'but what if I need it later -> consider the Alone winner who was evacuated for severe starvation, despite having around 30 days of food left, he could no longer assess his own needs), to possible binge eating disorder. He'll also certainly be dealing with re-feeding syndrome when he does access food again, and may need to be on supplements for this for a very long time. Magnesium, potassium, possibly others. Dwelling on food, thinking of where to get it, how to store it, what to eat, how much he can safely eat etc. may be occupying a lot of his thoughts more than you realise, especially since we need to eat and drink every day.

Due to food scarcity, I still have food hoarding issues I'm trying to unpack decades later. It becomes "invisible" because it's a thing most people don't notice, so it's a disordered part of you that gets seen the least.

* Hallucination/psychosis is less common due to stimulus - though not impossible. Would there have been animals or creatures or even plants that he could have "bonded" with? Or come to care for? Definitely falls under the "talking to himself" category.

* Re: Making connections, this will have been fairly significantly damaged after such a prolonged period of time. I mean, hell, people's ability to socialise after Covid lockdowns is still damaged, and we could talk to people online. And that was a considerably smaller period of time. You might find he makes more 'socially scripted' conversation that's correct, but then too quickly goes into subjects that are too deep and/or monopolise the conversation (he got used to talking to himself after all, no feedback!) He may also find that many of the mundane things people deal with as problems are completely unrelatable for a while, and that may make him feel even more isolated than when he was literally isolated (it is its own unique agony). Trying to make bids for connection only for many of them to fail or not quite happen the way he remembers them happening, because pretty much no one shares his lived experience anymore, making him a true outsider even around people he used to know well, is a very lonely experience.

* People who have been isolated + living in deprivation often experience symptoms of chronic illness in the long-term, if not permanently, due to malnourishment / starvation / lack of reliable water etc. Even if he does have all of these things, there's definitely a period of time where he doesn't know for certain how long it will last with the seasons etc. Bodies are impacted by this constant level of stress. You can probably expect - even with the best possible impossible levels of sci fi medical care - some degree of joint pain, functional somatic illnesses connected to trauma (migraines, stomach pains etc.), sleep issues. There might be meds to support that, but his body will never be the body it used to be, ever again. His body has learned deprivation now, and a body never forgets that. He may be more eager to conserve energy, even if he has more than he thinks he does. Alternatively, he may overspend his energy in his eagerness to connect, and not understand why certain things that used to be easy are now frequently or constantly exhausting.

I would definitely look into refeeding and food issues around isolation and prolonged isolation, and "scarcity mindset."

I know for myself, I would actually probably have him at first spend many weeks or months obsessing over who did this to him, and then that completely falling away over immediate issues like food, connection, shelter etc. It's hard to feel paranoid when you NEED connection with others. There may even come a point where he's like 'fuck it, I'd rather at least talk to the person who did this to me, than no one ever again for the rest of my life.' What you imagine he would dwell on heavily for 2 years is like... eh... people's ability to even think coherently starts to fall apart in the face of living leaner in isolation. His most serious and heaviest thoughts on this matter likely happen initially, and then disappear in the face of priorities - survival, connection. It might even feel like a luxury to start thinking about who did this to him when he comes back.

A lot of the time we imagine as 'boredom' while isolated in this kind of circumstance is usually actually spent resting or sleeping due to the constraints of less food / water / unpredictable circumstances / possible predators etc. He's gained the full-time job of survival, and it's not a 9/5, it's a 24/7. It's *really* hard to focus on like 'who did this to me, who can I trust' in those circumstances. Paranoia might even feel like a luxury when he gets back, or might even be an inconvenience that he just doesn't want. It's really important, imho, to get a grasp of how someone's thinking changes in spaces of prolonged deprivation. Paranoia can happen, for sure, but how someone comes out of iso in prison, is very different to wilderness deprivation.

A random recommendation is simply to watch a few seasons of Alone. It shows you how someone's mindset is affected over time, and also the things that they start to become incredibly worried about over time (infection, starvation, thirst, shelter). Alone Australia season 3 in Tasmania is also good because of the different attitudes / perspectives (they seem quite different to the US mindset). If you don't have time to watch entire seasons, watch the last 4-5 episodes of each, the people who have been out the longest, who are impacted the most. Look at the before and after pictures. The premature aging. Etc. It's probably the closest people come these days (in a way that's documented in an ongoing way), to the true mindset devolution into full survival focus.

Also, many of them do end up talking to themselves or the "camera." Even people who hate filming themselves. Many say at the end that the camera became "their only friend" and become attached to physical objects, like carvings they've made, or a stone etc. Many are still sick months/years later. Some get permanent organ damage.

I mean obviously you can write a story where he obsesses over who did this to him, but it seems like if he had to make a choice between using his spare energy for this, or for imagining connection with people, or talking to animals or plants, he'd choose the latter just from his personality? And these two things aren't super compatible. It might even be that his delusional paranoia starts when he gets back, and he finds that shocking and confusing, because it wasn't something he had to deal with for two years. After all, sometimes the worst mental health crashes happen *after* the traumatic event, and not during.