I have a few things to say here:
1) Spy Wizard, STFU you're stupid. You're clearly still disturbed based on your abusive childhood. Nobody cares.
2) This douche of an ex is just purposely doing what he can to make your life (and those around him) hell. It must have been an enormous shot to his ego that you walked out on him. Now he's living with his Mommy. I bet he's a very angry and messed up person. Sounds like a complete waste of skin.
3) I hope you get things straightened out through the courts. If you are sole custodian (hope you win) then YES he is responsible for paying child support (fuck off to those here who don't agree).
4) I've been through a crappy divorce/custody thing but my ex didnt have a pot to piss in, didnt have any legal representation, no job, no decent family to support him, no car, no license, no bank account, no house. I too left him and I believe he still holds it against me 12 yrs later, but not nearly as bad as he did the first 5 yrs or so.