I am sorry he is having trouble.
This can be an upper respiratory infection, an allergy or he may have hit his nose or inhaled something like grass or dust.
He may have cat flu. This is analogous to our common cold.It is an airborne virus so no direct contact is necessary
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 baby was indirectly exposed to other cats he likely contracted it from them or stress.
Feed him very odorous food so he can smell it and will keep eating. If he gets dehydrated, give him a few droppers of unflavored pedialyte throughout the day to hydrate him and balance his electrolytes.You can also offer tuna water drained from the can
He can have an OTC called clortrimeton - a quarter of a ten tablet once a day if he is congested. Or benedryl @ one mg per lb every 12 hours Both work for allergies as well.
You can use little noses pediatric nose drops if his nose is stuffy.
You can also take him in a steamy bathroom where you have run a hot shower to break it up. If he develops any colored mucus that means the infection has turned bacterial and will respond to an antibiotic.Some vets script one even if it is viral to prevent secondary infections like pneumonia.He should be seen if he refuses to eat or gets any worse.
Please let me know how he is feeling. If you get worried I will always be here to help further.Just press reply anytime to talk further.
I hope he feels better very soon.