Oh and you can't make him see the kids. Don't ask him when he's going to see them it may come off as you trying to control him and the situation.
Make a schedule you both can agree on --- I haven't done this yet. I've been a doormat and he just comes and goes as he pleases. It's something I need to change but I will say we aren't waiting around for him. When he chooses to be here we go about our business as if we never know when he will or won't be around.