I think he owes an explanation to you and your son where he was for a year an a half. While you cannot control him, if you are going to be doing the supervising, you deserve to know what's going on in his life. I would never let it go unsupervised unless you are court ordered to do so. Forgive me if I just missed it, but how old is your son?
My DD's are 8 and 14. While I have never said bad things about their father, he showed his cards himself to them. They deserved to know the truth about addiction and what they are dealing with. I told them what they needed to know in a very age appropriate way w/out bashing dear ol dad. I also sent them both to counseling which helped them very much.
Good Luck and God Bless to you and your son!