I'm so sorry that you're going through this. Get away from him and fast. He has a weakness that only he can overcome--no amount of action on your side will do it. Right now the only thing that matters to him is himself and the next time he can score something. He's too stupid to realize that he may not be getting "pure" stuff--some drugs are cut with things that, over time, can damage or even kill.
No matter how much you care, you've got to stay away from him--the last thing you need is to somehow innocently get swept up in it and have your good name ruined.
Good luck and please keep posting. You seem to be taking it very well but I can only imagine how deeply you're hurt.