Is there any discharge?  Or just sneezing?

Also, is he eating and pooping normally?

Finally, is he lifing his head to try and get more air in, or just making wet miserable noises?  The head-lifing is an emergency!  The rest are serious but if he’s eating and pooping okay then tomorrow may be fine.

How is he doing today?

My little guy is a sneezer. He sneezes often throughout the day. The vet said there is nothing wrong with him – it is probably hay dust. He does like to bury his head in the hay.
I also get sneezes/hacks from both my bunnies when the put their head in their food bowl before I finish pouring in the pellets!

hmmmm. That’s strange. Well, you could try sifting them a bit first since they seem to be a little more powdery and see if that helps.???

If it’s an allergy, then the papaya dust may be doing it. I was thinking pellet dust earlier, because sometimes both hay and pellets have dust settle toward the bottom of the bag, so when you come near the end you can easily scoop it into the dish. But your papaya theory makes sense.

You can shake or wipe his food before putting it in his bowls. However, if it is the tablets and an allergy, I’d just stop giving it to him for a couple of days. If he stops sneezing, then that’s it.

As for not doing it before, in humans allergies build up. So they can come on after all has been well before. I would expect the same can be true for bunnies.