Surprisingly if it's that (which it commonly is) then this is actually an easy fix.
It requires a perspective shift-
Op what do you hope for your boy to become when he is grown?
Surely, a happy person, a kind man, who is capable of healthy adult relationships and dates kind, happy women.
Sound about right?
How is he going to do that when his idea of what a relationship looks like is cold, and warped and painful. When all he saw was his father treating his mother shaudily...and his mother staying and taking it.
It is only in leaving that you do right for your son.
By giving him a happy home and happy mother. That cannot be achieved if you remain with his dad.
Secondly, as someone who witnessed her mother stay with an abuser (grandparent) I can't tell you how often I wished she would choose herself and get the fuck away from this person. How hurtful it is for a child to feel responsible for protecting their parent from an abuser.
That is how your child will feel if you stay.
That is...assuming they don't go the other way - and become an abuser like him. Do you want that? Your beautiful, warm, bright son...to be moulded into a cruel man like him?
You have a chance to save your child from this and raise him right.
To show him we do not stay with bullies. That we choose good and joy and light. You cannot do that in that dark home with that evil man.
For your son, leave.