I'm by no means and expert, but I wonder because of his diagnosis - which I imagine can be devastating vs. relieving for most people - he's feeling he like he's doing you a favor by not trying to have a relationship with you? I would guess if you walked away, he wouldn't follow. Of course, it depends on his personality too.
If you want to keep having a relationship, maybe just send him a short voicemail/text/email with a note saying I think you may have a lot going on but I just want to let you know I'm here if you need me. Maybe that's the validating message you need to show you care?
I can see that being a possibility. He has said to me a couple of times in the past that I'm best keeping away because he knows he's a toxic person, and that was before the diagnosis. He told me his therapist is telling him to stay home a lot and keep to himself and family so maybe he's just following their orders for once, instead of going out getting drunk and high (which is what he used to do). So I kind of don't want to pressure him too much because for once he might be doing the sensible thing and if he thinks being distant from me is for the best and good for his mental health, I don't want to mess that up. I guess you're right. I'll just let him know to contact me if he needs me but not expect too much from him right now, thanks.