Help!! My rabbit has snuffles!!!?

My rabbit is 4 years old and has had snuffles since Dec. He has been to 2 different vets and has been on 4 different antibiotics. I still have him on them. He eats really well, is very energetic and does not have anything going on with his ears or eyes. He had a nose culture done but it came back negative (because of the antibiotics?) The vet also stuck the microscope gl on his nose and that came back negative. If I take him out of the house for a sleepover, he starts to get better. His cage is clean, so I don't know what that is about. Can you please help??