Speak with your student in private. Tell him that you have noticed he has not been acting like himself, and provide specific examples of this behavior. For example, you can say, "When you are in class, you do not participate anymore. Instead, you just sit with your head down." Let him know that you are concerned for his well-being, and want to help.
Ask your student if anything is troubling him. Whether he is experiencing problems at home or at school, he may need someone to talk to about his problems. Assure him that you will not repeat what she tells you to anyone. If he tells you that he feels depressed or hopeless, encourage him to see a counselor.
Explain to your student how talking to a counselor can help him feel better. Tell him that a counselor is trained to help people express their emotions and develop effective coping methods. Tell him that he should never feel ashamed about asking for help, and that you will help him set up an appointment with the school counselor.