It seems the best solution like others have said is to sit down with him and your mom and talk about it. Don't let your mom give him any money and not let him have any friends over. If he wants to smoke pot he is going to anyway. Beating him up or yelling at him is not going to get him to stop. If you do that he will just loose all respect for you and continue anyway. Calling the cops isn't going to help either. Was it you that said you are studying criminal justice? If it was and he gets caught at your house you stand the chance of getting in trouble. A short term solution could be to make a bet. Its called the saltine cracker challenge. Bet him that if he can eat one saltine cracker while he is high that he will stop for one week. Its nearly impossible to do when high. I know its not exactly what you want but it could get the ball rolling.