I don't think I'd take my son to inpatient to see his dad - especially if we were divorced. His bad choices are not my problem anymore. My job is to protect my son.
First of all, dad chose drugs and alcohol over his son. And I am not going to put my son's emotional well being out on a limb for the possibility of a relationship with a father who has proven time and again that his son comes second to drugs and alcohol. I just wouldn't risk allowing my son to be rejected again. My ex would need to prove himself for a long long long time.
Also, my opinion is that there are bad bad consequences when someone chooses drugs and alcohol over their family and not having a relationship with your children is one of those consequences.
Just because he is in rehab, doesn't mean he's going to stay in recovery. Give him some time to get his act together in the real world before you judge whether he is serious about his recovery. Time is on your side. The longer you wait the better.