Flagler, although you find yourself in an seemingly intractable dilemma, the answer is really rather simple. Your wayward husband has to make a choice. Either his future will include the other child and not you, or it will include you and not the other child. Nothing else works. This OC is not your stepson. He is the product of an affair. That is what is meant when such a child is called "illegitimate".

married for 40 years
DS - 38, autistic, lives at home
DD - 37, married and on her own
DS - 32, still living with us