How old is he? Do you have other roosters who might have hurt him? Does he free range or get out where he might get into something dead or rotten? He has wry neck (torticolis, crook neck.) It is a neurological symptom. It can be from a neck or head injury, a vitamin E deficiency, and is seen in some diseases including Mareks and others that affect the brain. Most people use human vitamin E 400 IU daily plus some egg. B complex may help as well, 1/4 tablet daily crushed in food or a spoonful of water.