Can I be required to pay child support after my son turns 18?

My son lives with his father and is a senior in high school. He was held back a year in middle school due to behavioral problems and some learning disabilities, so he’s about to be 18 but just starting his senior year. I thought that my child support obligation stopped when he turned 18, but his father is saying he can force me to pay throughout the entire senior year and maybe even longer if a court determines that my son is fully intellectually disabled. How long does my child support obligation last, and what is needed to prove that a child is intellectually disabled?