He needs support. Lots of people with mental issues self medicate with harder drugs. If I were you I wouldn't worry about the drugs as a moral issue as much as I'd worry about the fact that he's probably struggling with a lot of awful mental health issues and uses drugs so he doesn't have to deal with them.
The more alone he feels, the more he is going to use. There probably isn't a lot you can do here, but not having family available to talk to will just make things worse for him.