Christopher brings his book to school the next day, and Siobhan reads it. She asks him about Mrs. Alexander’s revelation about his mother and Mr. Shears. He assures her that he’s not going to tell his father about it, and he’s not upset or sad about it because his mother is dead and Mr. Shears is gone, so the situation no longer exists and being sad would be stupid. At lunch he carefully avoids yellow foods, and in the afternoon he paints pictures of aliens.
Christopher seems to really not mind about his mother’s affair, even when Siobhan presses him. His logic essentially says that the affair happened in the past and nothing can be done about it now, so there’s no reason to get worked up. His rules, like not eating yellow foods, still hold. Nothing important to him has changed.