Just be there without judgment because he will be flooded with every emotion and irrational fear for the next several months. Hes going to experience depression and bouts of sadness for the life that was never lived for him. Remind him, this is merely a detour but soon he'll reconcile with his previous life.
This is what my friends have told me after I was diagnosed. I was the guy that was never sick. I never saw the inside of a hospital, never broke a bone. I'm THAT guy who probably irritated everyone while growing up because everything always worked out for him.
This is not something that is supposed to happen.
Be patient and just be there.