I agree with g.willikers that starting at home, before your range trip, is a good idea. Even before you start him with an airsoft pistol, you should work on teaching safety rules, proper gun handling, and operating the controls of whatever guns you have. Have him do some dry firing, and start teaching him the proper grip and stance. Doing a first lesson at home will be less stressful (and quieter!) than at a range, and it will let you spend more precious range time actually having him shoot.
And you probably already know this, but don't plan on doing any shooting yourself unless it's to demonstrate something specific. It's your friend's day, and all your attention should be on him.
Formerly known as Vanya.