Please make sure he is separated from the others! If it is contagious, you don't want the whole flock with it. Treat and feed him last to try to prevent exposing the others to his illness.
Is he eating and drinking well? How old is he? Nasal discharge? How's his eyes?
Have you introduced any new chickens into your flock lately? Do you show them? Has anyone that has chickens been to your place lately? If so, how is their flock? Have they had ANY respiratory illnesses within their flock within the last few months?
Continue the antibiotic for at least the amount of time on the pkg! Sometimes longer if the chicken hasn't improved, and sometimes they need a different antibiotic. Sometimes it's viral and antibiotics do NOTHING to help!