It sounds like it could be a respiratory disease. Is there any swelling of his face/comb and wattles, or any discharge? Even if there are no other symptoms, a respiratory disease is likely.
The disease could be viral, in which case all you can do is keep him warm, hydrated, and well fed. Offer him scrambled eggs, applesauce, and yogurt if his appetite decreases. Electrolytes and probiotics are good idea as well.
Just as likely, the disease may be bacterial, in which case you will need an antibiotic. To start with, you can get some oxytetracycline (Terramycin, Duramycin, Tetroxy HCA-280, and LA200 all contain oxytetracyline). If this doesn't work, try Tylan50 or Tylan200. The dosage for Tylan50 is 1cc for large fowl, .5ccs for bantams, injected into the breast muscle using a small needle once daily for five days. The dosage for Tylan200 is .5ccs for large fowl, and .1-.2ccs for bantams, given the same way as Tylan50 for three days. Tylan is a nice strong antibiotic, and should work if you are dealing with a bacterial infection.
Remember to isolate your bird, as respiratory diseases spread quickly and can be serious.
Hope this helps!