You mention that your son has "always been friendly, popular, and happy" and appear at a loss to explain why he has undergone such dramatic personality and behavioral changes. Your son's and your own need for professional intervention and therapy is long overdue. You must discover why your little boy has become so chronically depressed (if that is the correct diagnosis) and cannot seem to resurrect himself from this dark place. Clearly, something has happened to cause these attitudinal and behavioral changes, within or outside of the family. He is not going to "snap out of it." You need a therapist's help in determining what has caused your son's continuous depressive thinking and how you may help him return to being that happy child you knew.
Please locate and interview several highly recommended child or family therapists in your area who have dealt with depression in kids this young. Select the one you feel that you and your boy would feel most comfortable seeing. Then begin the journey of restoring your boy's spirit, optimism, and sense of well being. Good luck.