I'd say he's got some really weak tendons. I've never seen one that bad. Perhaps you can get some calcium rich foods into him and make sure he's the one getting them. How old is he? Has he always been like that? When my cockerels reach 3-4 months of age, if I feed them regular chick starter, they develop hock problems where they start walking funny. "Funny" in this instance is with really stiff toes, always having their toes fully spread out and also lifting their feet higher than necessary to take a step. Changing them over to lay pellets at that time fixed it but since I started using Flock Raiser (higher calcium) for my chicks I haven't seen this issue.