I wonder if the stress is exacerbating whatever is wrong? Causing temp
to spike and reduction in appetite. Maybe you should try him at home,
I would probably consider this if it were one of my cats.

Is it possible your cat had any kind of poisening? Did blood work
include toxicity tests? Lastly, could kitty have something viral that
antibiotics are not helping thus the reason for the continuing temp?
Are there any neurological symptoms?

As far as appetite goes I would ask your Veterinarian about the use of
Reglan (spelling?) it reduces nausea and perhaps the antibiotics &
stress are part of the appetite loss....I agree that if you can get him
eating perhaps things will start looking up...

Are they giving him any sedatives at the Veterinary office to "take the
edge off" Perhaps there are barking dogs and unfamiliar sounds and
smells and these could be startling and frightening your cat, I wonder
if sedatives (mild) would be allowed for him given his current status
and stress? Maybe they could add something to his IV?

If you decide to leave kitty at the Vet, can they put kitty in a quiet
area? Make sure cage is high up - not towards the bottom - cats feel
safer when they are up higher.

If you leave him there bring him some blankets and things from home
with your scent on it - maybe even bring his feeding dishes from home
and food. Another thing to try to get kitty to eat is using pureed
baby food - you can syringe this, they have high salt content but some
cats will eat this when all else fails. Also if you take kitty home
ask for some cans of AD and SD.

How old is kitty?
Is kitty vaccinated for everything including Leukemia?
Is kitty from a multicat household and if so, has your Veterinarian
discussed FIP with you?
What kind of Xrays were done? Of this throat/nasal area? Could
anything be caught in his throat/sinus/nasal area?
Does kitty have any dental issues?

It probably would not hurt to get a second opinion from a Veterinarian,
preferably one from a different clinic. Hope things settle soon. Jen