You can't take PR off your Ex because he has been violent towards YOU.
It's not you he has parental responsibility over!
Yes, he sounds like a twatbag of the highest order.
But that doesn't matter. He's STILL your child's father.
PR means that he has PARENTAL RESPONSIBILITY for your son, as do you. It's not something YOU get a choice over - if you chose to have a child with this man, then he has PR for that child just as much as you do.
You would be up in arms if HE said he was going to take PR away from you (not that he can, he doesn't get a say in it either, you are your son's parent, he is his son's parent, end of story).
You can't punish your ex for the way he treated YOU by taking your SON away from his father.
Your son isn't a 'reward' to your ex for good behaviour, or something that you get to take away from your ex as 'punishment' for his bad behaviour! Your son is a PERSON with a right to enjoy time with both his parents. Even if one of those parents IS an abusive cockwodge towards the other.
Stop trying to use your son as a 'pawn' to play silly games with. You will leave emotional scars a mile wide. I should know, my parents did the same over me.
Grow up, put your son first, accept that he has a right to continue a relationship with his father, don't badmouth his father around him (that's what MN is for!!), don't mess with your son's head in a twisted attempt to get 'revenge' on your Ex.
Doing that won't hurt your ex. It will only hurt YOUR SON.