I'm sorry to hear this. Does he have this year round or is it seasonal? He may have allergies and just have a stuffy nose. If so you could try putting him in the bathroom while running a hot shower, using a vaporizer, or using little noses pediatric nose drops. Some people also give their cats chlortimeton allergy tablets.

My Sky will also occasionally have this problem. His nose whistles too and his is most likely seasonal allergies. He'll also sometimes have sneezing attacks. I've tried giving him allergy medicine before and it seems to help. Good luck.