That seems like a classic URI, or upper respiratory infection. There’s a bacteria in his nasal passages that is multiplying and needs to be killed with an antibiotic or antibiotics. You’ll need to obtain these from a vet. The sooner, the better, so the bacteria has less time to multiply and grow stronger.

On this old post, I replied with a long comment outlining Wick’s 3mo dealing with a URI: https://binkybunny.com/FORUM/tabid/54/aft/157402/Default.aspx

… Make sure you see an exotic vet, not a dog/cat vet. While well intended, regular vets do not have the proper knowledge to treat rabbits, especially when URI treatment requires multiple potential treatment revisions and trial and error of medications.

… Allergies is also a possibility, but if you notice the sneezing ramping up and getting severe, it’s definitely a URI in my opinion. Allergies typically don’t cause the sneeze fits you’re seeing. Reducing dust in hay and litter will help not aggravate his nose.

What color is his nasal discharge at this point?

A URI may not start off having discharge. Imagine a human cold. It doesn’t immediately start with gross colored snot.

If the sneezing frequency ramps up, schedule an appointment sooner rather than later.