Wife's brother feeling mental problem.

My wife's brother was very good till the 10th standard. But after it, he suddenly lost interest in everything. Neither he talks to anyone. Only watches tv. Before someday, he started talking to himself. He tried to ran away two times from home. He was very good in every activity. But what happened suddenly to him is still unknown. I think he is in depression. He is not fine yet. He only answers what you ask. Neither he demands for anything nor he is interested in anything. He was also unable to clear his 12th in 2 attempts. He is the only son of the family. How can we recover from such condition. What is that problem. Kindly help us in this regard. We need help regarding this. Parents are still in tension. Please help. Please