I am sorry your boy is not feeling well.
He most likely has cat flu. This is analogous to our common cold.
All cats carry upper respiratory virus on their own fur. Cats that live together become immune to the carried strain but as soon as the bacterium is exposed to another cat it will infect them. If this boy was exposed to other cats he likely contracted it from them or stress. However, the virus is airbourne so direct contact is not even necessary.It is common for the immune system to weaken in older kitties. He could also have allergies.
Feed him very odorous food so he can smell it and will keep eating. Tuna, canned cat food, etc
Warm the food to bring out the aroma.
If he gets dehydrated, give him a few droppers of unflavored pedialyte throughout the day to hydrate him and balance his electrolytes.
Drain the water from a can of tuna into a bowl and let him lap it up.
He can have an OTC called clortrimeton - a quarter of a 4 tablet every 24 hours if he is congested. Or benedryl @ one mg per lb every 12 hours. I prefer chlortrimeton because it is a teeny pill you can add to food.For URI or allergy.
You can also take him in a steamy bathroom where you have run a hot shower to break it up. If she develops any colored mucus that means the infection has turned bacterial and will respond to an antibiotic
You can use little noses pediatric nose drops if he has a stuffy nose.
You can add 250 mg of lysine to his water to boost his immune system. Or use these:
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