Can my exhusband have me pay child support for our son who has been living with him for 6 month?

My ex -husbands pays me child support, and pays a 75% on the children s college tuition along with all medical co pays and insurance. MY ex-husband has claimed that he is disabled and has a reduction in pay. We now have almost the same income except that he has a rental apartment that adds to his income. He wants to take me to court to file reduction in child support because our son has been living with him so in turn wanting me to pay child support for my son to him ( my son is 19). Change the areas that he current is responsible for Medical Ins, Co pays, college tuition to a 50/50 split . We currently have joint custody of our children. We have been divorced since August of 2009.