If he does have nasal congestion, and therefore can't smell his food (which puts most cats off eating since their appetites are largely influenced by their sense of smell), you could try some steam therapy to help loosen things up. Hang out in the bathroom with him for 10-15 min while you run a hot shower, a few times a day if possible.
Also feed him the stinkiest foods you can find: sardines, fish-based canned Fancy Feast (the ones with shrimp are particularly fragrant), low-sodium canned salmon.... whatever it takes - don't limit yourself to cat food.
But yes, a vet visit might help figure out what's going on, in case it isn't "just" a URI.
If it is, try adding L-lysine powder (from the health food store or pharmacy) to some wet food (500mg twice a day). That will help prevent the virus that causes most URI's from replicating.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler