Well, corn isn't good for chickens and layer feed isn't good for roosters. He definitely need more food than free ranging or this wouldn't have happened as easily.
Cam you get chick grower, meat bird, all flock, etc? Feed that too all your birds and just leave oyster shells out for your girls.
What's his diet? It looks like trauma brought on wry neck.
Give him 400i.u vitamin E capsule and about a third of a B complex tablet every day for a few weeks. Any brand from the grocery store.