Butterfly, you just listed everything you did and the big fat nothing your ex did to help your son. How can you feel responsible? It makes no sense. If your son is living with you you get to make the rules not the ex. Tell him to take a long walk off a short pier. Jeez.
As a mum of two adult children I will say you need to sit your son down and tell him what your expectations are. Time for him to find a job and start paying his fair share.