Form my experience they use the sub to lesson the withdraws but can use the same day. Detox is always hard and that is why the go back to the drug. the thing is once they are detoxed they need to say away from the drug. But as windmills has said: leave that to the addict and not focus on that part of it. You need to work on you and how you handle the addict. Not so much on there problem, more on life in general and how you two connect.
You need to have rules / boundaries with them and not allow them to walk all over you. I recommend you get it in writing and then it is clear. There recovery is theres and ours is ours to deal with. Focus on you and what you need, they will follow suit. This is a life long disease and he must deal with it on his own!!!! And he will, if you just let go and let God.
Have you been to Nar-anon for your own help?