If my son is on disability and his father and I have joint custody... is he still required to pay child support?

HHow old is the son ? He may still be required but at a reduced level depending on your state and circumstances.

Calif is right we will need more information to give you a better answer. The state is very important as well as the age of your son. Each state has it's own laws and yes, they do very sometimes by quite a lot.