LOOK THIS IS TRICKY but its never going to change because he lets them get away with it, and if u ask him to do something about it, and he actually does, he will end up blaming u because it will be you that made things difficult for him.
ive been through this, but i dont have children. He should be ashamed of himself, letting his family react to the children in that way. You need to stop this and see how selfish he is being by 'trying to keep the piece' your kids are your angels and they should be his too enough to demand that his family gets to know them for who they are.
honeslty, get away and let a real man into ur life. its gna be hard but yea. and if u leave and get back with himn in the same year, forget it, things wont change. and ill tell u why, because you are really the one who iss allowing this to continue, ur letting him and his parents rule the roost. get rid of him.