What kills a kitten (or cat) with URI is that a cat won't eat anything it can't smell. Kittens are especially vulnerable because if they can't smell Mom and their special nipple, they'll go off their feed and that kills a rapidly growing kitten very quickly.

If he stops eating, it will be because he can't smell anything. If the really cheap, stinky tuna cat food won't tempt him, then syringe feeding is in order. He won't thank you for it but it can pull him through the worst of the illness. You'll know he's better the first time he actively sucks on the end of the syringe.