I feel that what your son is experiencing is abandonment.
I experienced that a month ago, and truth be told.. I can really say that loneliness and abandonment nearly killed me.. it's a miracle that I'm still here typing this stuff . xD In any other situation, i would already be six feet under. What you should encourage your son to do is join some group activities where he'll be able to find someone to keep him company, as well as ensure that his days are spent happily and that every day brings with it something more to look forward to in the future.
Being studious might also be part of it. Since his best friend is now gone, he has no motivation to study because it's quite likely that he spends his days alone, without anything to look forward to.
Now, he needs something to look forward to because he won't want to be living for no reason.. he needs to smile, and I hope that I can help you bring his smile back.
I know how he feels because a person dear to me sort of left my life last month, and i went through the same symptoms your son did, except that I did try to kill myself. I don't know how I'm still alive. xD
Willing to help your son out and I hope that I can return his smile back to him..