Hopefully you're keeping him secluded from your other cats, then you won't have to worry so much about fleas etc. Is there anyone else you can pursuade to foster this guy that could afford to properly treat him? If you can't afford the medical treatment he needs, as much as this sounds cold, he would be better off not suffering. It is wonderful you rescued this guy and I wish you the best of luck finding him someone that can care and love him. Good luck!