My ex is behind in child support for our eldest son that lives with me.

Unclear from your question is whether your arrears have been reduced to judgment, how old the kids are, who has lived with whom for how long, etc. -- those are the things that would determine the answers to the questions you ask.  Presuming you are owed money, you might end up with an "offset judgment" through which the money owed to you is used to "pay" whateveer you are owed, until it is exhausted.  You probably should consult with a family law specialist, however, as it sounds like there are several layers to your problem, and all the facts are not clear.