I would not feed flax seeds to chicks and only very little to adults occasionally (black chickens get a wonderful green sheen), because of the amount of cyanogenic glycosides it contains.
Look at his legs and feet: In case they are feathered, are there pinfeathers coming in that might pinch him with each step he takes?
Look at his legs and feet: In case they are feathered, are there pinfeathers coming in that might pinch him with each step he takes?