Hi, I just have a few comments. You mentioned your son is 'afraid' of his dad? I take it that he is afraid of losing his support or worse, his love? There may be issues in the sense of what your son saw (or heard) and learned between you and your ex, but more too likely he is 'daddy's boy' and needs Daddy to show him the proper way of becoming a good person. IF that is possible. If Daddy (your ex) dismisses and invalidates you or your concerns and your son 'simply follows suit' then maybe Daddy needs to re-think his position in all of this and give son a bit of serious guidance before he develops beyond the anger and acting out part.
In a similar situation, I saw a boy take quite well to a positive female role model (his teacher) and she was able to see his pain and work with the boy and his issues. Finding that ally can be a tough go, but someday your son might thank you. At the point he is at, and his age, he might be diagnosed as ego-centric and is acting out his hostility. Underneath that he is in pain. At the moment he may be diagnosed as having a conduct disorder.
My suggestion would be to make your self aware of issues in that category, and make notes of when or if he has ever been cruel to animals (pets for example) or to other kids. Counsellors tend to minimize things and rationalize them away as being 'typical of a young boy in his position'. Don't let them do that. It serves no one to sweep the issues under the carpet.
You might also try to make note of any and all disturbances at school and also try to stay focussed on what things he enjoys doing, hobbies etc as they might provide a distraction.
In another case, I worked as a volunteer at a local YMCA with troubled boys in my neighbourhood... that might be an option, or if he is interested in sports. Somewhere you will find he will connect with something that can help to provide an outlet for his anger. For my son, it was hockey.
Those are just a few of my thoughts, I hope you find them helpful.
Bill