Hi (again) unigeezer .I should add that Morty also has had runny nose and sneezing...maybe he stopped eating because he can't smell his food?
I talked with another vet who said that he had a cat with a similar problem and what he did was add 10 parts of water to one part Afrin and with an eyedropper put a drop or two into the affected nose and he said that cleared it right up or at least allowed him to breathe easier and smell his food
Yes, perhaps he can't smell the food and that might explain the inappetence.
I'd caution you about using Afrin, though...first, because its taste is so awful that it causes profound drooling in cats - you can read up on it here: Oxymetazoline (Afrin®) for Dogs and Cats
Secondly, note the potential for a 'rebound' effect if it's used for more than ~3 days (as humans experience!)
Here's a reputable reference for you on common respiratory issues - and, you'll see a link there for nasal drops which provides a much more palatable option than Afrin: Harpsie's Website - cat flu (FVR and calici virus)