It's really hard to say at this age. Many times, vets will use the sub-q fluids to flush the kidneys and this will give the pets a better shot.
At this age, thyroid disease, renal failure and liver failure are all concerns.
Since he is not eating or drinking, I would be very concerned.
Any time that a cat won't eat for more than 48 hours, you are in serious trouble. Their liver can fail and then the cat can die. It is called hepatic lipidosis where the liver thinks it is being starved and starts "grabbing" fat cells...and actually ends up killing the liver.
I suggest that you see the vet first thing tomorrow for a full exam and possibly bloodwork to find out what's going on.
It's great that he has finished the antibiotics and that his fever has been reduced. But now you need to work on the problem of him not eating. You might want to talk with the vet about using some neutrical or another viatmin source during this time.
Let me know if you still have questions.