I am sorry for your brother's loss. You really need to just let him grieve. Let him know you are there to talk if he wants to talk or just to sit with him quietly if that is what he wants.
One of my older brothers friends has died. The boy who died was only 12, and was shot. My brother is not coping well. I too am shaken, but how can I comfort my brother?
One of the most important things you can do is be there for him. It can be very confusing to anyone who has to deal with a loss at such a young and tender age. Try praying with him. The bible tells us that "God is close to the broken-hearted, He saves those who are crushed in spirit". Remember to be patient and kind. Hopefully soon he can move pass this horrible experience.