Never give up, never stop supporting him. It sounds like he does want to get clean, but then the addiction draws him back in. Addiction is about pleasure, so something about the drug is enjoyable, even if it is just for a moment. That's why it has a hold on him, physically or psychologically. Help him remember all the reasons he does not want to have this destructive substance in his life and remind him that he can do this, that the people who love him believe in him.