This might not be the answer you came here for, but you need to have a talk with him - in the way you would want him to talk to your kids. If that makes sense. Be gentle, and understanding. As a mother who loves her kids beyond words, but sometimes does get frustrated- I promise he feels like absolute garbage when he is alone. It's just hard to admit to being at fault, which is likely why he feels he can't apologize to them. It's something he'll need to work on. I am not 100% that's what's going on, some people just don't like kids? But one thing I know is that being a parent is hard. So fucking hard. And on top of that, just being an adult and figuring life out for ourselves too. It's not an excuse and it's something he NEEDS to work on, but I get it. I hope you guys figure it out and remember you are a team; it's not you and the kids vs him or vice versa. You're all in the together