Poor Sky. It does sound like a URI. I hope he gets better soon and that your other kitties don't catch it.

Of course I know you have no way of knowing but I wonder if he had one as a kitten. Jazz had a severe URI as a kitten and she has had a couple of flares of it since (she just turned 2 years old last month). My vet told me this is very common if they had one as a kitten. He said the older she gets the less flares she will have and then when she gets to be a senior she may have trouble with it again. Maybe that is the case for Sky too.

Get better sweet Sky!