Let me offer a few comments.
The good news is, you don't need to prove the 'wilful' element. Rather, your ex needs to proove that he does not have the ability to comply. In order for him do so, he, most likely, will need to testify and waive his 5th amendment right. Once he does, you (your attorney) can cross examine him.
Someone suggested that you use the state agency becuase they're free. However, contempts are extemely difficult to prevail upon, and to succeed, you really need to have extensive discovery completed before. This will not be done properly by DCSS. And, once tried by DCSS, and, if lost, you will not be able to charge him again with the same missed
installments again.
Don't give up. IF he becomes collectable, and IF you remain persistent, you can collect what's owed to you.