He could possibly hurt himself?

You aren't being a bad friend, that's for sure! You know that he's going down the wrong path by getting involved with drugs and other bad things like that. All you're doing is trying to help. This is all because you care! Maybe just let him know that...? It's his choice about what he wants to do in his life. You can't force him to stop. I think all you can really do is let him know you're there for him and you want things back to the way they used to be at least. Like when he used to tell you everything and was honest.