Why doesn't Jimmy know his dad?

Jimmy has never known his father, as his dad, Crab Little, has been in jail since Jimmy was five. In fact, Jimmy lived with Mama Jean even before Crab went to jail because the older man was not equipped to deal with a child after his wife's death. When Crab suddenly shows up outside Jimmy's apartment, he does not know how to deal with him. Jimmy goes on a cross country trip with Crab, but does not understand why Crab wants him there or what he expects.