My children lost a friend that very same day, in a car accident on 544 (accident caused by the rain).
My kids are 11, 9, almost 6 and 3. The younger two aren't really "sad" b/c they don't quite get it -- but it has been very tough for our 11 & 9 year old (as well as many of the other kids in our homeschool group who knew the dear sweet girl who passed away).
We have just tried to let them cry, and answer their questions as best we can. They have been mourning, and are angry, but we are just going through the process and making ourselves available.
Like you, we encouraged our kids and a few kids from our homeschool group to get together so that they could talk about it with each other, and take comfort from each other.
We are getting our kids a memento , a stuffed animal like their friend had, something for them to remember her by - a happy memory that made their friend happy.
I think all that you can do is just be available. Hopefully at some point he will talk, or maybe he will just talk to his friends and counselors - but that is better than nothing. Just be there for him if he cries or mourns, just be there for him if he is angry, and just encourage him to celebrate the priveledge of having been a part of that child's life.
We also had our Pastor come over and hang out with the kids, that meant a lot to them.
I think that you are doing a great thing by being aware of the situation, being aware of his confusion and pain, and encouraging him to be with his friends. I think you are doing a great thing by being there for him and not pressuring him.
I pray that your son is able to get through this difficult time as "easy" as possible. Take Care & God Bless.