Ok bare with me... I have had some kind of psychotic episode most likely brought on by smoking some powerful weed (not doing *that** anymore!). For a couple of days I had an intense sense of paranoia. It freaked me out completely and I felt truly on the edge of sanity where people were spying on me, following me, etc. It's been almost a year and while I am not longer freaking out I feel that I have somehow opened a door that I can never truly shut again. I will see someone and somewhere in my brain I will think they have a strange or suspicious behaviour. But it's a background thought (if that makes sense) and I don't "believe" it to be true.
When the episode I had happened, I could still think of how irrational my thoughts were and the situation was. What actually helped me was to not care. To actively set my mind to not caring about whether I was or wasn't being observed. I also coupled this with the reality that I was an incredibly average individual and the cost of tracking me would just be astronomical for someone who's got really nothing special going on in their life.
So of course my episode was likely drug-induced but the fact of reasoning through the psychosis could be helpful. It seems you do not want to bail on him, and that's commendable of you but do watch out that in trying to help him you don't fall down the rabbit hole with him.
Perhaps retracing the psychosis he's experiencing back to those 8 years, what happened around the time he started thinking he was being stalked?
If he's open to the conversation talk about 'confirmation bias'. There's plenty of evidence that we do this constantly and it's a leftover from our evolution - the quicker we could make associations between an event and a threat the more likely we would avoid actual threats. 'A shadow and rustling noise = danger' VS the modern version 'a red car = danger'. We've evolved a brain to recognise patterns and our brains can sometimes go into overdrive.
Well that's all I can think of right now, the key is whether he is open to talking about it. If you take it as a matter of fact kind of way it might go down OK but he needs to be open to the idea that we have vulnerable and malleable brains and we need to care for them in the same way we do our body and overall health. If he is not open to talking it might just be out of your reach to help him and it could be worth contacting family if he has any?