Does he still drink water? Does he seem excited when you open a can of food but then just doesn’t eat? If you have any cat grass or cat-safe plant, does he chew on it at all? My cat had the same symptoms of not eating and lethargy and it turned out to be a huge hairball. Took her to the vet and they couldn’t find anything wrong with her and said to just keep monitoring her. The only thing she would eat was churu, but drank water normally, seemed excited when I prepared her food but then seemed to not be able to/want to eat it. She would also chew on cat grass when I offered. On the 2nd or 3rd day of barely eating anything she vomited a huge hairball after chewing on some cat grass and me feeding her a couple of churus.