Can you clarify?
When the decision to attend the pricey school was made, were you already divorced? If not, was anything put in the divorce decree?
Did your H pay for some of son’s years there, and then suddenly stop paying during the senior year?
Can your ex afford this cost or was this one of those short-sighted decisions that he made w/ little thought as to how it would get paid?
Maybe you can ask the school to contact him and demand payment.