I feel for your boy....life as a teenager is hard enough but dealing with the death of friend, a sudden death no less, can be excruciating. I was 16 when a dear friend died in tragic circumstances and it really turned my world upside down.
I did find solace in my fellow students and our close group of friends but looking back I wish I entered some sort of therapy. We were offered counselling but of course at the stage of our youth we baulked at it and rather stuck together and bonded over our grief. To this day I feel that quite a few of us are still trying to deal with issues related to that one day which changed our lives.
I say give him some space to grieve the way he wants and let him know that you are there to support him and to lend an ear if needed. Maybe in the near future, after a year or so, mention some counselling because a neutral person for him to talk to could be beneficial. Sometimes teens can unknowingly suffocate each other with their overwhelming emotions so it may be good for them to talk to someone who can just listen unbiasedly.
All the best x