What does Bruno do at the end of Chapter 8 to make himself feel better?

Bruno wrote his grandmother a letter.

That day he sat down with a pen and paper and told her how unhappy he was there and how much he wished he was back home in Berlin. He told her about the house and the garden and the bench with the plaque on it and the tall fence and the
wooden telegraph poles and the barbed-wire bales and the hard ground beyond them and the huts and the small buildings and the smoke stacks and the soldiers, but mostly he told her about the people living there and their striped pyjamas and cloth caps, and then he told her how much he missed her and he signed off his letter 'your loving grandson, Bruno'.