If you haven't noticed any improvement just giving him supportive care, get him some antibiotics. It might be a bacterial respiratory disease. I would recommend oxytetracycline (like Duramycin, Tetroxy HCA-280, and Terramycin) if it doesn't seem too severe. Otherwise, get Tylan which is one of the best antibiotics to treat respiratory diseases. Tylan50 is best for chickens, and the dosage for it is 1cc for large fowl, .5ccs for bantam, injected into the breast muscle once daily for five days. Use a small gauge needle. You could also use Tylan200, but try to get Tylan50 first.
Hope this helps!