If your son lives in your home, then you need to enforce the rules of your home. Which hopefully would be no drugs in the home. If your son supports himself, there isn't much you can do. In times like this it is important to stay emotionally close to your child without monetarily supporting him. That means you spend time with him but, do not give him money. Parents who give their addict kids money are supporting the habit and taking away natural consequences. You cannot stop him from using drugs, you can keep it out of your home.