The most important thing IMHO is to let him understand they realize he is 17 or 18, and that he might be nervous. When he accepts that it will calm him down. Tell him they already know he is the top of the top, so that is in his favor.
Second, follow his lead, if he wants to be silent in the car let him, if he brings up anything discussing the interview, talk about it, otherwise ask him where he wants to go out for lunch.
The interview committee typically asks the questions about why X service, when did you decide, what will you do if you get no appointment, and current events. As far as the candidate book, or reading about famous AF people, etc., I wouldn't waste my time unless it is something you want to learn about. They don't expect you to know the Airmen's Creed or the 1st graduating class from the AFA. They are looking at you from an overall perspective, they want to know that you are the most likely candidate to be commissioned in 14. They know that in 1 month from getting there you will have the Airmen's Creed down pat.