I'd keep a very close eye on him, as the more experienced owners will tell you, they go down very quickly.
my own experience is that one of mine has sneezing fits, lots of tiny sneezes in quick succession but no discharge. He has a tooth which grows very close to his tear duct, and the tear duct is actually damaged so lots of things can set him off, so now I know, I keep an eye on him.
It could be a number of things - if you have a good vet then maybe call them first? or if you are worried, which you obviously are, I'd just take him down for a check-up in case its the start of somehting.
hope he is ok