I don't know what it might be. Keep him isolated from the other birds so he won't spread germs around. Keep him warm. One technique is to cover one end of the cage with a towel and shine a lamp on it so the heat goes into the cage. Another technique is to put a heating pad under half the cage. You only heat part of the cage so he can move if he gets too warm.
It's often very helpful with sick or injured birds to give them an electrolyte solution. The easiest way to do this is with a sports drink like Gatorade or a product made for human babies like Pedialyte. You can also make your own - the simplest recipe calls for sugar, salt and water:
http://www.webmd.com/hw-popup/rehydrati ... ar=hw86827There's a slightly more complicated formula here:
http://clearingmyemptynest.blogspot.com ... emade.htmlTalk to a vet if you can.