Suggestions:
Sunday best.
~ Khakis, buttoned down shirt, dress shoes; boat decks are fine, no sneakers. Wrinkles from sitting is fine, wrinkles like they pulled it out of the hamper, not fine.
Groomed hair
~ He doesn't need a military cut, just don't enter looking like it is not kept. DS did not have a buzz cut, his hair was close to touching his shirt collar. He got every MOC nom. It just looked like it was kept.
Current events
~ Be prepared for off the wall questions, i.e. what do you think about Benghazi, should the military have been involved
Academic questions
~ We see you have an A in AP Calc, but got a 650 on the Math SAT, why?
Be prepared for off the wall questions, such as favorite book never assigned to read for school.
Most importantly as a parent, our DS did a shut down mode during the drive. He listened to his Ipod. We didn't pepper him to prepare for the interview. They know what is at risk, you know your kid. If he wants to talk, talk, if he wants to be silent, be silent. Follow their lead.
One last thing we did was made a day out of it. We took DS out for lunch to his favorite choice, pre-determined. We also decided to hit up Best Buy because Santa had already come, and use his Xmas gift money. In your case, use it as an early holiday shopping spree.
We did that so he felt less pressure about the interview, more about the joys afterwords, regardless of how it went.
You just need to show it is just another day. No biggie. If you can manage that he will be that great kid the school, coaches, friends all talk about! This is more about your attitude and how you handle than you know.