He sounds like he is grooming your son to meet him.
Is the restraining order against son, or just you?
I think an honest chat is needed with your son about what his father has done, the consequences of those actions and that his father is grooming him to believe he has changed etc (telling son what he wants to hear). If father really wanted access, he could apply through the courts, be assessed whether he was still a risk, and get supervised visitation etc.
At 12 your son is still confiding in you, trusting you with this dilemna... at 14,15,16 he will be meeting ex without your permission, ex will spin his own version of how hard done by he was and wanting DS to move in with him to get back at you. If DS has a bad day because you've made him do homework, help in the house, get off the games etc, he'll be easily manipulated by ex.
You need to be honest with your son, and put the onus back on father. Tell son, if father wants visitation he will need to get it legally, as the judge ordered it - if he can't be bothered to do it, then that is his choice, not ours.