Crispin: Cross of Lead by Avi

Set in 14th century England, a 13-year old boy known simply as "Asta's son" must flee his village after the death of his mother. He doesn't know why, but someone falsely accuses him of theft and murder. This in turn has him declared a "wolf's head" which means anyone is allowed to kill him. While on the run, the boy finds out his real name is Crispin, and the mystery that has surrounded his family begins to unfold. Also read the sequel Crispin: At the Edge of the World.