What can I do to help him through his grieving process? He will not talk much, but I do know he feels he did not do enough for his friend. His school gave him no grief counseling; in fact, when he started crying, he was told to go sit in the closet until he was over it.
Your son was never afforded an opportunity or the level of support needed to go through the necessary emotional transitions that accompany a healthy adjustment to such a horrible event. He also hasn't been able to forgive himself for not doing enough for his dying buddy. In his mind, I'm sure he believes that his perceived lack of support for his friend made his friend's dying more painful. Your son has most probably been depressed for some time and has been in need of professional therapy. His school's lack of grief counseling was unfortunate and insensitive, but his teacher's comments to "go sit in the closet until he was over it" were inexcusable and cruel.
You need to see to it that your boy gets some skilled, compassionate therapy right now. He is showing everybody that he is still too overwhelmed by his best friend's death to function in a healthy and productive manner. Search for therapists who have counseled boys your son's age on similar issues of grief and loss of a friend. He has shown you that he cannot and most probably will not get better on his own. Dealing with the normal roller-coaster emotions of a teen his age is difficult as it is. His additional burden of intense sorrow and guilt has made the emotional load too much to bear. I know that you'll get your son the professional help that he needs.