It depends on what he has, as to what you use to treat it with. Have you added any new chickens recently who might have been a carrier of infectious bronchitis or MG? There are a handfull of respiratory diseases, each with various symptoms. If there is only sneezing, it could be infectious bronchitis virus or just an environmental irritant. Do you hear any rattly breathing or see any bubbles or swelling in the eyes? A virus will not respond to antibiotics. If it is something like MG, it might respond to Tylosin for pigeons available online, but I would try to get some testing.
