He is probably sick take him to the vet to see wut is wrong
My Puppy Has A Runny Nose And Loss Of Appetite, What Does This Mean?
If he's sneezing frequently, I'm going to assume that he has a cold. If he's really lethargic (lazier than usual, sleeping a lot, slow-moving), I would suggest just checking in with the vet, to make sure he's not seriously ill. I doubt that's the case, but you can never be TOO sure!