Today I noticed Wilbur has been sneezing, slowly getting a bit worse as now he just had a sneezing fit. His eyes and nose are clear, no fluid around them. I did switch his litter to (untreated) wood shavings yesterday. Do you think this could have something to do with it?

has he had his deworming shot? as for the wood shavings, untreated shavings still can give of gases that we cant smell that are harmful to the rabbits liver!! I only use Kiln dried wood pellets

Only aspen shavings are safe, but untreated pine shavings are not safe to use for bunny litter, nor are cedar chips. I've noticed that my local pet stores are now selling bags of "wood shavings" and it's hard to tell which wood, but assume it's probably pine and really cheap stuff. You can use untreated wood pellets, however, and pine is OK for that. Feline Pine or untreated wood stove pellets, though my favorite is Aspen Supreme pellets. I use Aspen shavings, which absorbs odor very well and might not bother him either.

If he's better, then try another kind again. (sorry)

Oh, most pet stores will let you exchange an opened bag for a different one. They sell to finicky cats, after all. Take back returns all the time.
And Aspen is a kind of wood, not the actual brand name. There are a couple of Aspen shavings manufacturers.

Ok, I did take it out of his cage and just put in hay.. but he's still sneezing and now has a slightly wet nose . I think i'll ring up the vets and just ask to speak to Wilburs vet to see if it's worth him going in for an appointment!