I have one that does this also.... but mine is going after a tattoo (very small butterfly) on the outside of my ankle. She seems to think it is just too yummy looking to leave alone. I have to be very careful and keep an eye on her whenever I am wearing shorts out in the run.
First you need to figure out if he's doing it out of fear or aggression. If he's doing it out of fear, reassure him by holding him awhile and maybe offering him treats to establish trust. If he's doing it out of aggression, very lightly thump him at the back of his head but don't hurt him. You need to assert your dominance and show him that you are the top roo. Discipline/character building starts in the brooder, not after he's grown.