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Cats do not have whims like people. Whenever they stop eating they are either ill or in pain. If he has no other symptoms like fever, diarrhea, constipation, infection, etc then there is something going on that is difficult to ascertain - but it exists nevertheless. Sometimes a cat can have an abscessed tooth or root that is in the gum.They can ingest a piece of plastic that in not perceivable in an xray. They can be mildly dehydrated or have a metabolic or organic imbalance that is too slight to read on a test. He may also feel nauseated.
If your cat is not diabetic begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.
Boil chicken or fish as the scent of food cooking awakens the appetite sensor in the brain.
Please get started ASAP and let me know how things are going. I will stand by to hear from you and help further.