Many upper respiratory infections in cats are viral, and are usually self limiting, however if they continue affecting a cat long enough other invaders, such as opportunistic bacterial will compound the original infection. That is when antibiotics are needed.
Since this has been going on in your cat for a week it is probably time to take him to the vet.
Additionally, if your cat sniffed up a foreign body and it is lodged in his nose it may have to be removed by a vet, and could be the cause of his excessive sneezing. Sneezing is the bodies way to remove invaders.
It sounds like it could be the cat flu. If so, then yes, you probably do need to see the vet.
I'm no expert though so it is best to wait for the vet's response.
J