I'm sorry for your situation.
The only person that can help him is himself. The only thing that anyone else helping him is going to do is prolong his ability to remain involved in drugs. Helping him is not preventing his death but simply delaying it. The only chance that he really has is if he is allowed to reach his bottom and then seek help. Now, I think that that is the time to actually help....but in healthy ways.
With all of that said....we all just do the best that we can and only you can make the decision of when to let go. Sending you throughts and prayers.