It could be a number of things, The bedding he lives on could be very dusty, something could be dusty or annoying his nose when you let him out. Sometimes Naz will get into corners of moms shelves when we're in the living room and since they are a little dusy he'll sneez once to twice. So it could be just a dust or other moment seenzeing issues.
Then it also COULD be he's sick and has a Resp. infection. Is there anything else you can tell us? what type of bedding he lives on? how often you clean his cage?? He could have allergys and the list goes on.