As someone else said, test him and set up boundries. It's not easy to do especially if one of the consequences is that he can no longer live with you.
He wants to use because he's a recovering addict and will do so if he thinks he can get away with it. AlAnon is a great program that will teach you a lot about not only yourself but your son. A large learning for me was identifying between helping and enabling...not always a clear delineation.
Best wishes to you and your family and welcome to SR.