what financial obligation do divorced parents have to help pay for children's college expenses

I have three children with my ex husband. Two are in college. The ages are 23,21,18. He paid child support for all three until the last one just graduated. Divorce papers said support continured until 18 or graduated. he cooperated even after the first two graduated High School because I explained that they have college expenses. now he has decided that he should be done with child support although there are still college expenses. What rights do my children have to receive financial hel from him, he makes 6 figures a year. I am the primary custodian. I have four children total. With the loss of 1200 a month, I am finding it difficult to pay help them financially. Also, he has strained relationship with oldest two girls but has a relationship with our son.