If your order says that he gets to claim your son every other year then he gets to do it every other year. He probably never gets a refund because the state takes it for his arrears. But that's not going to help him catch up on 10,000.

My ex is now over 12,000 behind. Last year he did taxes and had like a $200 something refund - and he didn't get any of it. Why? The state took it. I didn't get any of it either so they may have kept it because the mother of his oldest son was on government assistance before he found out he was the dad.