Welcome to the forum
So you’ve said possibly to fight a URI, so it seems that these courses may be more of a precaution rather than because of a clear cut and diagnosed URI.
How long has each course of antibiotics been?
What antibiotics have been used?
Have you seen any improvement with the courses of antibiotics?
Is his weight stable at each weight check?
URIs are obviously bacterial and if there is bacteria present but symptoms don’t clear or they only improve a little then it can be because the wrong antibiotic is being used (I see youve said three different antibiotics though), the course isn’t long enough or it isn’t a bacterial issue at all so consequently you aren’t likely to see an improvement on symptoms.
What did the vet find when his chest was listened to? Was his chest clear or was there any crackling etc?
Sneezing on its own is not necessarily a sign of a URI - it’s commonly a sign of airway irritation - its also not the first sign of a URI in the same way a viral cold affects humans.
Piggies have narrow airways so any hay dust can and will cause sneezing and irritation in the passages.
I see you’ve tried other hays, are they dust extracted? Have you tried shaking any dust out of the hay yourself?
Have you ensured there are no irritants in the room - candles, air fresheners, perfumes etc. Such products can also cause airway irritation.
Try adding a bowl of steaming water next to his cage or taking him into a steamy bathroom. It may or may not help relieve symptoms and therefore can be an indication as to whether nebulising would be useful.