What I suggest is that you follow your own excellent instincts. If you could have solved your wife’s problems with relationships, you wouldn’t be divorced. Your son is now in the same abusive situation you were in that led you to leave. Your son, being a kid, doesn’t have the same impulse control that you do. If your wife is violent with him, chances are he will respond in kind. Punishment from you will only verify for him that violence is an appropriate way to deal with conflict. What is hopeful in this situation is that no one likes what is going on.
I encourage you to find a licensed marriage and family therapist (MFT). MFTs are trained and experienced in seeing the multiple layers of family dynamics. They don’t take sides. They work to understand everyone’s point of view and to make recommendations to make the situation better for all. If one or more of the people who are involved have mental health issues, they address that as well.
Start by going by yourself. You and the therapist will discuss how to bring in your ex-wife and your son. Please listen to yourself — and to me. You cannot solve this alone. If you could, you wouldn’t be writing to me. You have nothing to lose by talking to someone who has helped other families like yours find a happier way to live.
I wish you well.