Holden Caulfield is suffering from the severe depression and is socially incompetent which enables him to not form lasting relationships with friends or family. This comes from being alone all the time not sharing his feelings with anyone. He constantly feels betrayed by society as Mr. Spencer who told him about his expulsion from Pencey Prep. Holden felt the adult world was full of phonies and that innocent, little children were being exposed to profanity and phoniness. He believes it is his duty to save the children as Phoebe from entering that world (catching all the children from falling of the cliff). Also, he finds comfort talking to his ten-year old sister, Phoebe, whom he feels is the most intelligent of all. This kind of behavior tells that Holden consider himself alienated from society that no one understands him but a ten-year old.
I consider his kid-brother, Allie's death, triggered the mental illness. He partly feels responsible Ally's death and doesn't let go of it as the holding of the baseball glove. Holden exhibits the usual signs of mental deterioration as performing irresponsible acts as running away. His motives for his behaviors are triggered by betrayals or "phony" actions. If left untreated in his condition, he will feel the necessity to be completely isolated from the world therefore causing to perform even more irresponsible acts as harming himself by getting excessively drunk. I prescribe he immediately start seeing a psychologist who is involved family therapy and involvement with the outside world as a job and a tutor for his education.