Catholic, Lutheran, and Jewish Social Services all have counseling on a sliding scale. As a child and teen I was abused emotionally by my father's wife. My father did nothing to stop it, which is abusive in it's own right. I finally had an emotional breakdown. My mother brought me to a counselor. The counselor had a talk that went somethng like this: "Courtenay will no longer be going to your house. You may see her anywhere and anytime you would like to see her, as long as it is not at your house and your wife is not present. If you feel the need to fight this in court, I will testify on her behalf." He knew that he would not win in any way with abuse documented by a counselor. SO he agreed. I do not know that that would completely work in this case, but I would STRONGLY suggest that you seek free/sliding scale counseling for your son. He will need this for quite a long time, if my experience is any indicator!
There is a LOT of crap that goes along with a father who doesn't care about their child enough to intervene in an abusive relationship being perpetrated by their spouse. I would strongly suggest that while he IS with you, you surround your son with MANY strong positive male influences--guys who love him because of who he is, not because they are blood relations. When he grows up, this might help him come to the realization that there were men who loved him because he is special, not because blood "required" it of them...and that his father is the one who lost out.
DO try to seek out counseling for him. There are many sources of low cost or free care, you might have to look for them, but they ARE out there. And they WILL stand up for your son, if it is necessary.
Getting taped proof that the behaviour is occuring can be very helpful, by the way, if you can figure a way of doing it. We finally got my father's wife's psychotic behavior recorded when she started pranking our house more than 60 times a day for years on end. We finally figured out who it was when *69 was invented...and then reported her to the phone company, who got the cops involved, who tapped her line and ours, and then the courts got involved. Free of charge. Not that this woman is making prank calls, but that if you can get proof, it can get other people involved who can help you. Your standing up for your son will in the long term be a very VERY good thing for him emotionally. I can not stress enough how damaging this can be to a child or a teen ager.
This would be the case, by the way, were it perpetrated by a mother, a father, a cousin, OR a father's wife.