Depression causes a chronic sadness, fatigue, exhaustion. It also makes it hard for a person to be motivated to do things, even things they once enjoyed. This is sometimes hard for people to understand. Another thing is a depressed person cannot just "snap out of it" or "cheer themselves up." You can help your son best by being loving, caring, and supportive. The other thing that will help him if you can do this is professional treatment from a psychiatrist, and therapist. We are also here to offer our support to you and your son. Best of wishes.