If he's living with you, you can tell him no drug use in your home.
IMHO, it's an uphill battle with pot use as it becomes legalized in some states. Young people are growing up believing there's nothing wrong with it b/c it's legal in some states (and just a matter of time in other states) and "not addictive."
My house my rules, I don't care how old you are!
qwer