It is hard to say without reviewing the court order(s). From the information you provided, the arrears appear to be for periods before he was incarcerated. If that is the case, when he gets released he will owe the money and they will come after him for it. In other words, if the arrears were established for periods when he was not incarcerated, he owes them.
If you found this answer helpful, let me know by clicking the "Mark as Helpful" button at the bottom of this answer or select it as Best Answer. It’s easy and appreciated. This answer is provided for informational purposes only and not for the purpose of providing legal advice. Comments made on websites such as Avvo.com are provided for information purposes only, and you should not base a decision to act or refrain from acting based upon this answer. The only way to determine how the law may apply to your particular situation is to consult with an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation. That relationship is established by the execution of a written agreement for legal services. Also, see Avvo's terms and conditions of use, specifically item 9, incorporated by this reference.