What could be wrong? Stray cat, 4 months old, high temp, vomiting and lethargic?

just keep him quiet and warm. Do not give Milk, cats cannot tolerate the lactose in cows Milk. Try feeding smelly fish (cooked.) for him as well, to whet his appetitie. He is on antibiotics, give them time to kick in, but if he keeps vomiting, get him back to the vets son as you can. Give him clean water and do not feed on dried biscuits either, this will make the problem worse. Hope he gets better soon.