He absolutely does not have to, but not wanting to this strongly is uh... odd. Especially if this person is going to be in his child's life.

I'd encourage him to put the past behind him and move forward in a healthier manner. If they can all find a way to be cordial, that's going to be the best possible outcome for the kids. Tell him he can hate them all he wants on his own time, but he's a big boy now and needs to figure out how to drop whatever he is stuck on. I recognize I'm being harsh towards him and this could also easily be an issue on the other side as well, I don't know the situation. I just know it's best for the kids if ALL the adults could figure out how to grow up a bit and move forward.