First shot would be your state child support division, or whatever it's called in your state, and every state has one. They're really pretty good at what they do, if a valid support order is in place, they have no problems working across state lines, and they are cheaper than attorneys - like free.
Employment information is required to be submitted to them within 20 days of hiring a new employee. At that time, the state should be applying for wage withholding. This is supposed to be automatic, but better ask, anyway. Max withholding for child support it 50% of wages after taxes*. He's really refusing a six figure job to dodge support payments
*A couple of states go to 60% in the event of long overdue payments, and unemployment is not considered a valid reason for being overdue.