Child Support and College Contribution are similar but different. You would need to file, presuming there's no prior order to the contrary, a petition for college contribution. You would need a second count to collect on back-owed support. He can be compelled to pay what he receives each month, even if it can't be directly attached. But that will be up to judge.
Your ex can be compelled to pay child support arrearages. Child support terminates at 18, graduation from high school, or the end of the high school year in which the child turns 19. Contribution to college expenses is provided for under a difference section of the Illinois Marriage and Dissolution of Marriage Act and provides that each arty will pay according to their respective abilities to pay. If your ex has remarried, his new spouse's income will be taken into consideration when allocating college expenses That;s the good news. The bad news is there is new case law that requires a petition to be on file prior to incurring the college expenses so if you have't yet filed anything, you need to rush to an attorney to help you with this issue as it may already be too late.