You are right to ask yourself "What IF he starts using again?" Do you have any boundaries FOR YOU about the kind of behavior you will and will not accept in your life, the kind of person you want to spend your time with?
What are you going to do for yourself if he starts using again? How many times are you willing to help him detox before you decide enough is enough?
Because nothing you do will make him stay clean. Nothing. He is a man. He is an addict. You cannot fix him. You can only protect yourself with boundaries should he decide to use again.