If four vets haven't found a problem, he may just be a sneezy pig. There may be an allergy to something or an irritant in the environment that you can't pinpoint if it's a new behavior in a pig who didn't have this issue before. Have there been any changes (new bag of hay, new laundry detergent in use, scented candles, live Christmas tree in the vicinity, etc?) If it's a new piggie who has always been sneezy, they may just be sneezy. If it's any consolation, one of my pigs, Leela, was very sneezy when we got her, so we got her vet checked and she was fine, just sneezy. Later on she developed an eye infection and we had that checked and I once again asked them to go over her respiratory tract because she still sneezes a lot, and again she was fine, just sneezy. I suspect she has some sort of allergy but can't pinpoint what it is (she does sneeze a lot outside in the summer and develops watery eyes in the lawn, so I think it may be some sort of grass/pollen allergy and since she lives on hay, it's hard to remove that from her environment!