I am surprised that his attorney is not encouraging him to sign the termination in a voluntary case. Is there something else like back child support that is still due?
Since you requested the hearing it should follow your motion so be sure you can prove everything in your motion. You are requesting termination of withholding.... judge will ask you why. If you said the child was 18, take a certified copy of his birth certificate. If you said he has graduated from HS then take his diploma or a certified copy. Take your original CS orders. Print a copy of your CS payment history to show you made all your payments. You should be able to access this through the attorney generals website.
If he is not entitled to the CS payments then you should be able to get them back. You might find that having an attorney would be beneficial in this instance. If you cannot afford one, the local bar association may have reduced fee or pr bono attorneys that can help you as well.