Depression is a serious condition.
Perhaps you could encourage him to see a psychiatrist for medication and a counselor to talk to. If he does not want to talk to a psychiatrist, a family doctor can prescribe medication for depression.
Depression causes people not to want to engage socially or do the things they used to do. Try to encourage him to stay connected with other people and to do things that he likes whatever they are- hobbies, sports, etc.
What To Do When Someone You Love Is Depressed : A Practical, Compassionate, and Helpful Guide by Mitch Golant Ph.D. and Susan K. Golant (Paperback - Mar 15, 1998)