Contact the Child Support office and request that they review your case. They will need to verify that your son has actually finished high school, but they can put a definitive ending date in your file. Once they discover their mistake, they will return the funds to you. Please note that the system has it's flaws; sometimes you have to seek a court order to terminate the withholding order and sometimes they reimburse and expect payment from the parant who received the excess funds. The Child Support Office should be your first step, though.