Just be his Mom, but remember he's still a sick kid to a point, so don't let him take advantage of you. Keep your boundaries, (no he's not coming home, etc.) but your not their to fix him, he is their to fix himself with the help of the doctors.
Remember nothing you do is going to make or break him or decide whether he is going to use again or not. So don't put pressure on yourself, just be Mom..
I don't know what to say or how to act when I see him.
Maybe ask yourself what or how would you want your family to treat you if you were sick from drugs/alcohol and they were coming to see you?
Sometimes if we put ourselves in their shoes first, then it makes it all the easier.
:ghug2