My ex is hurting himself, what do I do?

Ok so after we broke up (pretty sure it's not because we broke up) he started smoking pot and dealing it as well. That really bothered me for a while because we both felt the same way about not wanting to smoke pot (I don't mind other people doing it, I'm just not interested). I knew that that was his life lesson to learn though, so I stayed out. Oh and he also started treating people like crap (including me) and doing bad in school. Anyway, so I stayed out. The daily routine of him asking me to smoke pot with him and me saying no happened; but then I noticed his wrist. He had cuts, two day old maybe. He has had a history with depression and a really bad relationship with his dad that was one of the main reasons, but he never has hurt himself before. He has had thoughts of suicide before, but never got past writing a note. I know I should stay back, but it is so painful to see him do this. I really care about him so much. He means sosososo much to me. (I'm at that point where I still love him, but I will never get back with him) He was my first everything if that helps make sense of why I care about him so much. We're both almost 18. Our break up wasn't harsh, but he started acting rudely afterwards for god knows why. Anyway, we're not really friends anymore, so I can't really find out whats going on. I don't know what to do, doing nothing sucks and I would never be able to forgive myself if he killed himself and I had done nothing all this time.