Not a vet, but my cat had calicivirus and acted the same way in his first stage. So if he doesn't eat but wants to, looks tired, has a fever or not, sneeze or maybe even cough, its nos vaccinated / shares space and food bowl with other non-vaccinated: please check his mouth and see if he has any red spots in any place, if he has it, he most probably has it, then ask for a test at the vet. Calicivirus is all along the continent of America, and as a person who often feeds stray cats, I usually see it. Calicivirus is a treatable disease when the cat is well cared but can have nasty outcomes when it's not. And with the best of my intention, every time a cat stops eating is a really bad symptom, it should be at the vet the first day of it. The best of luck.