So your X went out and tried on some big boy pants. Good for him. Too bad he had to roll the legs up so far and wrap the belt around twice to hold them up. What a total TOOL.
Like others have said - Look how great you are handling everything without the person you just knew you couldn't live without. Yes, your house, your home, your life all working out so well - and he can't find any other way to act out over his pityful life than to try dragging the welfare of the kids in and witholding his obligations. What a total TOOL.
Do what you know you have to do in the courts. See if they can make him cover some of the filing costs while they're at it. In my state if they don't pay and you push hard enough, they get picked up and have to cover the debt to get out of lock up. If he ain't happy now, see how he feels missing payday and showering with Bubba. He knows these things. He has heard others tell their sad story. He is just pushing the limits because he believes he is above doing the right thing and responsible to no one - certainly not you.
It's all up to him and NOT your fault; unless you let him. You let him step on you with his left foot, he's going to want to use both feet as fast as he can. Boundaries!
Enjoy school, enjoy the house, enjoy the kids and living your Better Life.