Cats are prone to allergies just like humans. There may be something in your home that is irritating his nose or lungs. Remember, he is closer to the floor, couch, bed, etc. than you are and smells things up close and personal.
There is a possibility that he may have picked up a slight cold which should pass in a few days. I the sneezing doesn't let up in a couple of days take him to the vet, there may be something blocking his breathing passage that he can't clear himself.