What are you trying to treat? What are the symptoms? A respiratory disease or symptoms? Antibiotics can be used best if you know what disease they have, and that might require testing. There are at least 8 or more respiratory diseases from viruses, bacteria and mycoplasmas, fungus, and others...
Any pictures? Does he have a bad odor around his face? Have you added new birds or had a respiratory disease in the past? Do you have any antibiotics? If you cull him, your state vet can test him for diseases along with a necropsy on Monday am. Keep the body cold inside a cooler in garbage bags...