Ugh. Active alcoholics are selfish to the core. I think you already have the answer as to what you should do. You just need some encouragement. Keep repeating the three C's....
I didn't cause it
I can't control it and
I can't cure it.
I think its a great idea to move in with your in-laws! It will get you out of the house, being there probably brings a lot of anxiety. Your son will be much better off living away from the negative environment AH is causing. He already knows grandma and grandpa's house, and they can be a lot of support to both of you. Its a wonderful idea, go for it!