Final answer:
Andy's attitude toward Jo changed the most.
Step-by-step explanation:
The boy's attitude toward Jo that changed the most is Andy's attitude. In the given information, it is stated that Andy got into a fight with another boy named Lucas. Andy teased Lucas about being a big baby, and eventually provoked him into hitting him. This aggressive behavior shows a change in Andy's attitude toward Jo, as he went from being outgoing and friendly to engaging in physical violence.