I have this exact problem with a new baby we rescued from a pet store.
Do you notice any discharge from his nose? Or runny eyes?
The vet our bun went to first put her on antibiotics which cleared up the discharge from her nose and her runny eyes. But she still has sneezing fits, not as often but they are definitely still there. So we took her for a 2nd opinion. Like your bun she is showing no irritation in her nose and there is nothing visible to the vet. Key word, visible to the vet. The new vet has decided to flush her nose with water to see if she has something stuck way up that is causing the irritation. I would talk to your vet about having this done.
We are also concerned about something in the house irritating her – so in the meantime, shake the hay out before you give it to her to get the dusty particles out as good as possible. Same thing with litter if you use a litter that could cause irritation from being dusty. We are investing in a humidifier also as someone else on BB (I believe Jerseygirl) suggested.