Wait... What??? :err:
You never mentioned that he had trauma in his past before. That changes everything.
EVERYTHING.
Really, I'm shocked that you didn't think this was relevant.
If he is medicating PTSD with street drugs, everything is a thousand times more complex. He needs help, but he won't want to get help as long as he thinks he can keen it under control himself.
He will probably not be able to just stop. Not only because of addiction, but because stopping would bring back the symptoms of his PTSD. ANYTHING is better than that. There is nothing you can do to him that would make living with untreated, unmedicated PTSD worth it.
I suggest you go get advice from a profession. Go see someone yourself, and get advice on how to help him. This is not the kind of thing you can fix on your own, and it's not the kind of thing you can learn how to deal with on an internet forum.