Wow that's a really old rooster, and he looks great. My observations:
He is walking with a pretty normal and symmetrical gait, alert, interested in food, clearly, but unable to hold his head in a normal upright position. I would suspect that he has an injury to his cervical spine or the muscles around his cervical spine, but not affecting the transmission of impulses to his legs etc. so not what a "central lesion" - stroke, damaged spinal cord, etc. would look like. He could have some cervical stenosis (narrowing of the outlets of the nerves from the spinal cord causing pain) or muscular injury. He's pretty active, considering. Aren't animals amazing? If he's eating and active and otherwise himself, I would doubt any problem with the gut, etc.