Hi Zenial, I don't think the court would look favourably on you taking off just to keep him from sharing custody, but if you had a legitimate reason like a job offer or to be nearer to relatives, that would probably go over better.
I understand about the MIL thing. My DIL's first partner probably wouldn't have give a hoot about seeing her, indeed he moved far away, didn't pay support and pretended not to be working so he only owed a miserable amount. It was his mother driving the custody attempts.
Another thought about you moving is that he's much more likely to get his son for longer periods like school holidays. At least with you living in the same town you'll be within reach, and it's only likely to be overnighters. You can also keep closer watch on the AH for obvious drunkenness or neglect.
Keeping an accurate factual diary about his drug and drinking, along with photos of his stash etc may help with getting supervised custody. Start being a little bit cunning.