Child support ends at the end of the 18th year. If you have an agreement for a longer time period your ex is just being kind. And if your son is 21 and hasn't started his junior year yet he shouldn't be supported. He should learn to make his own way as most of us who graduated at 21 did.
The child support obligation in New York by statute runs through age 21 unless the child is emancipated earlier. Your child support will end automatically at age 21, unless there is an agreement otherwise. Many separation agreements or divorce stipulations provide for payments for college costs until the child graduates or reaches a specified age beyond 21. Unless you have such an agreement with your ex that provides for payments after your son reaches 21, the courts do not extend the requirement to pay past age 21. This is true even though your son will not have graduated until he is 22 or 23.