I am sure that you have articulated your appreciation of his kindness. Continue to tell him how special he is, while providing him with opportunities to talk with you about his sadness, anger, and profound sense of loss. He will have times where he will become overwhelmed with grief. You can probably see those times coming and try to be very understanding on these occasions. But there will also be times when he gets blind-sided by a tidal wave of grief. Prepare him for this and explain that this is normal.
He is choosing to stay connected to his best friend by staying close to his friend's family and by acting as an older brother toward his friend's sibling. This role might be difficult for him to maintain both emotionally and pragmatically. Don't wait for him to come to you with his grief. Parents often assume that if their adolescents do not come to them with problems that they don't have them. That's an unhealthy assumption. Your son needs you to stay connected to him and his life, especially during an emotionally devastating time like this. If you see your son retreating for several weeks from his regular social life and the things that have brought him pleasure in the past, I think that would be cause for you to consider some professional counseling for him. The burden of his grief is astonishing at this time and he may very well need and benefit from some professional counseling. You might also talk with the parents of his friends and discuss how you, as a group, might best help your children through this dark time. Let me know if I can be of any further help in the future.