What information does Bill want his mother to know?why? Chapter 10-11

A letter is received from John, who writes of taking care of Rebel prisoners. It is the end of 1864. While he is feeding them, he discovers Bill is among them. Reluctantly, his Captain allows John to visit with his brother for a while where there are able to update one another. He tells his family that Bill has aged quite a bit and that he was not among those that killed Tom at Pittsburg Landing.