You can either try to enforce the divorce agreement in Supreme Court, or try to go to Family Court to adjust support (including the college expenses).
If he doesn't want to be reasonable and work out an agreement with you, then maybe he needs to resume picking up the kids from you and returning them to you (aka, "sorry honey but you can't drive your car on dad's weekends"). That might encourage him to cooperate and be flexible, so that you can return to being flexible with him.
Regardless, take your agreement and any divorce papers to a lawyer near you for a more detailed assessment of your case.
Answered on Oct 01st, 2013 at 3:46 PM