Do I still have to pay for health care after child has moved out of mothers house?

My son recently moved out of his mothers house. He is nineteen and living with his grandparents. He doesn't have a job yet.
My divorce states that I have to pay for my children's health coverage until the age of 21.
My ex-wife keeps telling me I have to pay for a family plan. Can I change it to a single plan and only cover my daughter?