Mites can be anywhere on your chicken, so look at the skin for signs of dirt or slow moving tiny bugs. Permethrin garden dust or 10% spray, or Sevin dust are commonly used to treat them. Here are some good links to read about mites in chickens:
http://www.backyardpoultrymag.com/laura_e_john/...
Is it hot where you live? Could he have been kept from water by the others? Is he having any diarrhea or blood in his poop? He is a bit older for coccidiosis, but that could also be a problem. Coccidiosis is treated with Corid (amprollium) in the water. Have you wormed your flock recently. I...
Can he raise his neck up, or is it droopy and limber, or is it stuck like in wry neck? Could he be weak and dehydrated? Has he been injured? Some diseases I would worry about would be botulism from eating a toxin produced in dead animals, maggots, or rotted vegetation. Also mold in the feed, so...