blessed-
well, there is not much you can do if he wants to not agree. i wonder if he is angry, blaming you (as per normal) and is now just trying to cause hassle to you. don't take the bait and get sucked in. let the lawyers handle it.
are you still considering a "nesting" situation with this man?
from where i sit, you have bent over backwards to offer him a fair, generous even, situation where he can get on with his life and be a father to his children. you are behaving like an adult, caring for the children, and even him.
he, in turn, is angry, wants revenge of some sort and wants to play a game. the stakes are high in his game. try not to get drawn in.
my best to you,
naive