So glad you got your cat in for help!, there are very nasty feline viruses, especially if he has not been vaccinated. (see about that after he gets well)
I can sympathize that Dr visits wreck your budget, most vets are very good about small regular payments. Try meat baby foods for him, and the juice off cheap oil-packed tuna. Since they evolved from a desert background, cats are not real good about drinking enough water even when they are well, make sure he has access to fresh clean water always.
Care, luck and love, I bet he'll be fine soon. Good Luck.