In the book Among The Hidden, Luke is the forbidden third child. His mother is fearful for him because if he is discovered, he could be put to death. However, Luke has discovered a relative life outside of his hiding place and wants to live as a real person. This is frightening to his mother because of the consequences if he is caught, both for him and his family. His mother tries to persuade him to stay hidden, and since Luke has no intention of following his mother's advice or bowing to her fear, he turns away from her by turning his head to the wall. This turning away symbolizes his wish to follow his own desires, and his mother leaves when she realizes that Luke plans to do as he wishes.
He was starting to get emotional...perhaps he had tears in his eyes.