If it is Newcastle, antibiotics do nothing. It's a virus and no virus responds to antibiotics. With those symptoms, I'm afraid I'd euthanize the poor boy. I believe that there are two forms of Newcastle. Exotic Newcastle is a nasty contagion and must be reported to the state here. I don't know what is wrong with him, of course, but what did the tests tell the vet? Did he say or just give meds?