Hi hvert, I'm not so sure that his behaviour is about either you or the friendship you've shared. Perhaps it's not so much him deciding on a "rational" basis that he doesn't want to tell you......doesn't want to talk about...........but more so his difficulties are stopping him from doing that.
Perhaps if you look at it more as, at those times..........it's the paranoia, delusions, diagnosis (??) that are speaking/acting/guiding his behaviours not him. Not the person you've held close as a friend. And there's nothing to say that that person-your friend- isn't still in there, it's just...........And maybe now he needs your friendship more than ever.
Maybe you can help him, try to be there for him as much as he'll let you even if that's just a very small bit. Although it does sound like he can do with some (a lot of??!!) encouragement to get some more/extra professional help. I don't know...........you'll know him personally so......maybe he'd respond better to a big push to get some help or maybe it needs the "softly, softly" approach with the paranoia?????
But then again, sometimes there's really only so much you can do.
And IF he is REALLY struggling with his mental health........and you haven't seen it as much DO NOT fault yourself!!!! Sometimes people struggling with the kind of things your friend has been struggling with can be so GOOD at covering things up/making them look not as bad as they are. It can be both an effect of their mental illness and coping strategies in trying to manage it.
And if YOU want any support then you know where we are.

Alison