It’s hard all around and will probably be a long term commitment if you decide to try and help. I don’t mention that to scare you off, just to prepare your thinking for small moves instead of thinking there’s a quick fix that will make things instantly better for him.
For me (in your situation) it was difficult to decide if it’s more important that he have someone he trusts (you) or that you try and get him help. Ultimately, if it’s possible, you want to lead him to realizing he needs help as opposed to pushing him that way.
In your situation I would ask questions about the strange things he believes, starting with asking if he’s willing to talk about it. Remember that he doesn’t think these things because they’re logical, he thinks them because something is amiss. Proving he’s not being targeted to yourself won’t help, he has to work it out for himself on his terms (and in terms he understands) and then take the extra step to recognize that something must be wrong for him to believe those things.
Once you know a bit more about what his needs are and hopefully can at least get him to admit the things he’s fighting with may be untrue, then you can try to negotiate a first step. Maybe that’s meeting with a professional, maybe it’s just setting up a time to talk every few days, maybe it’s moving back somewhere closer if that would help. All progress is good.
It’s exceedingly difficult to get someone help against their will. This is probably a good thing overall but it can be frustrating when you’re watching someone lose the life they’ve built. If you truly believe he needs help to get through this, try to find a way to show him that so he can make the decision himself. Make it as easy as possible for him, find resources in his area and have all the information handy. If you can call them beforehand (and he wouldn’t feel like you were sharing too much about him) then even better. Who does he call to make an appointment? Can he go now? What are their hours? How much is it?
Finally I’d just add that you need to make sure you’re OK with all this as well. It can be draining. Take care of you as best you can and get help with the parts you’re not comfortable with. Ultimately you have to remember it truly is his responsibility, just don’t use that as an excuse to give up when you don’t want to. Just don’t give more than you can lose.
Good luck.