I think it depends on the drug. If it's too severe , like Meth, there isn't a lot you can do until they hit rock bottom. You can try an intervention with the people who love him but I've never personally seen that work. I have a brother who is a Meth head ( and anything else he can get his hands on) and whenever anyone tries to help he takes advantage and steals from them. Some people just can't be helped.
Good Luck, I hope he gets better.
Your not just talking about weed, I hope.