Hi and Welcome,
I think you need to be sure of your boundaries with your son. You need to establish for yourself and for your son, what lines will not be crossed. If you won't allow him to use drugs while he is staying with you, then you need to be prepared to do what would be necessary if he does. Getting sober takes a lot of motivation and it's great that you are helping your son, but as you said, he needs to do this himself.