He should be tested for Toxoplasmosis.It is diagnosed by the presence of T gondi in the blood:
Hemobartonella also causes a high fever:
He would have pale gums and a low PCV (packed cell volume)
Of course there is the dreaded FIP:
Run a hot shower to make a steam room in your bathroom. Take him in for ten minute intervals to break up congestion.
Drain the water from a can of tuna into a bowl and see if he will lap it up. If he does, offer the tuna.
Warm human baby food chicken or turkey with no onion or garlic and lets see if he eats that.
Ask his vet about doxycycline
Please let me know if he eats and how he is doing.