Unfortunately, I have no answer for you, but can offer a shared experience. My little boys are now almost five months old and have been with us since they were six weeks old. Rascal has sneezed since the day I brought him home. No other signs of respiratory distress, no runny nose, over the top energy, and just as mischievous as his name. The sneezing always puzzled me and at the time the only thing I could point to was that he eats too fast.
I realized about 3-4 weeks ago that the sneezing had virtually stopped. (slaps forehead) Of course I immediately asked myself what changed about that time and the answer came back that I switched to new hay. I previously used orchard grass from SPS (husband is wildly allergic to timothy) AND, it was last year's crop so was a bit dry and I guess, dusty. None of my other cavies have ever reacted to orchard grass. But then I always say, "cavies are as different as we are". Not even Rascal's brother (Rafi) had a problem with the old hay to my knowledge.
The new hay is KMS bluegrass which I used some years ago, and just decided to give it another try along with their pellets. These boys are absolutely WILD about this hay and eagerly wait until just before bedtime when I tidy the cage, fresh water, fresh pellets, and a clean litter box with fresh hay. Interestingly, just now as I write this, Rascal just sneezed a couple of times. Oh, well.......so much for my logic! However, the frequency has been greatly reduced.
I wouldn't be in a rush to take him to the Vet based on your observations. If he has no other worrisome symptoms, i think I would just keep an eye on it.
