The union sends him out on the jobs but the union has nothing to do with the payroll processes of each company that pays him. A garnishment order would have to be issued by the court or the state to each company that he works for and it would have to be done before he gets paid.

The trouble with that is that he's likely to have moved on to the next company by the time the previous company is served and then it will be too late.

You're going to have to haul him into court for contempt or see if your state's child support enforcement agency can wring the money out of him.

If you get the state involved the state may have a way of intercepting his tax refunds if he gets any.