Before you go to vets you can clean his nose with a damp cotton wool pad use just clear water or I make some herbal teas for that, chamomile, thyme or just black tea, it can be warm or room temperature, not hot. Do it a couple times a day and if it is not gone by tomorrow maybe vet will help him better.
My rabbits sneeze occasionally and in most cases it's from dusty hay, I clean their noses and shake a bale of compressed hay inside a large plastic bag after removing it from package so just trying to reduce dust as much as possible. I am allergic myself so getting sneeze from hay sometimes as well.
Another question is why has he dirty nose check his bum maybe he has diarrhea or something and needs some assistance. Or digging somewhere is he indoor or outdoor rabbit?
Anyway if you clean his nose and see if he has it again in a few hours you can find out why this is happening, hopefully nothing serious, good luck!