He counts the number of people leaving the observation room. The speech therapist makes note of this. Both patient and therapist are carefully watching one another, sensitive to one another. This is the key lesson of the film. This is what great actors alert us to.
Life is about listening, about paying attention.
Because the therapist is listening, because she really cares, the patient is aware of this and begins to respond. The therapist is trying to understand the story the boy has to tell. She is a detective trying to find out various clues and put them together to develop her understanding and her ability to help him. Instead of freaking out, she relates to him on his terms and he is willing to let her keep the knife. When she brings into his world another therapist to help out, he is at first upset.