If your ex will not voluntarily pay--and you certainly can send him a certified letter or other communication asking him to honor his obligagtions--then the way to enforce the support is through the courts: you would sue him to enforce the order, and after (presumably) winning, if he still doesn't pay voluntarily, you could attempt to collect by executing on his personal property (having it seized and sold to pay), putting a lien on real estate, levying on his bank account, or in most states (not Texas or North Carolina), garnishing wages.