Is your husband good to your son, when you are not around? I know how hard this is, I have an alcoholic daughter, and already one of the kids moved out to live with their dad, who is also one, but he seems to maintain a better household than DD does, and it has broken all of our hearts. But, the DGD is such a handful, she might be better off with her dad and firm hand, which he does have. Time will tell. The other DGD is staying with mom, but she is of completely different temperment, and very good for DD, so I hope this might be a turning point.
You have to do what you have to do; and I know from experience staying and letting your son see you mistreated, might lead him to grow up the same way. It's a documented fact. My DH was raised in an abusive alcoholic family, dad was a week-end drunk, worked made a good living, but their week-ends growing up and vacations were a nightmare. DH was never one to drink after seeing what he did, or very little, but I know he is "damaged" from it. He made a remark to me one time that I never forgot. He said, "you have no idea what it is to pull your dad off of your mom and not know if your mom is going to get up." I never forgot that, and luckily that is one thing our kids never saw as they grew up. they all agree they had a pretty good childhood. So, hard as it is, remember what DH said and don't let your son see anymore of the abuse. If his dad is good to him otherwise, and you can trust him not to drink when he sees him, then your son will probably benefit more from that type of relationship than the one of being with him 24/7 and not knowing what is going to happen next. Don't hang back about supervised visits either, if your husband is not to be trusted. He will learn to control it or he won't, but the choice is his and his alone.
I admire you VERY much. Your life is your own, you only owe that little boy a good life, and yourself. You picked a great career and you will never have to worry about a job . . .start getting your ducks in order and take it one thing at a time, and then go for it. I think its wonderful you are seeing someone to find out the best way to go about approaching your son too, sometimes we do overkill and want to "tell them everything", when usually kids are just content to know a little and then go on with their little selves.
Good luck and keep us posted!!!