You may get more helpful advice over at the Divorce forum, but here's a quick list of things to look into:
1) Find out if your state has a child support enforcement service. This takes the pressure off you considerably and lends some oomph to the communication regarding what he's legally obligated to pay. You tell the service, they go after him. And after him. And after him. He doesn't get to ignore them for long before he loses driver's license and a whole other fun bag of things.
2) Contact your attorney. It's possible that a simple letter saying, "Dear Jackwad, Pay up or say hi to the judge. Signed, Your Worst Legal Nightmare" may be enough to wake him up. The cowards choose silence, and those are usually the same cowards who freak when they get something on official letterhead that indicates the jig is up.