My kids brought a copy of their resume as well since it provided more info than the app.
As for dress, my son wore business casual - khakis and button down shirt with a v-neck sweater or a blazer. He had a couple of suits from doing debate but I thought a suit was overkill and he didn't feel comfortable wearing it for this occasion. But if your son wants to wear a suit, and that makes him feel more confident, make sure it fits well and is a classic shade, such as Navy Blue or Charcoal/Light Grey. Wear a traditional white or pale blue shirt - no black shirt and purple tie.
Having judged a lot of high school debate tournaments, I've seen a lot of misfitting suits (because they were bought off the rack and had no tailoring, such as is done at a Jos Banks), and odd color choices, like black and a dark shirt (they looked like they were attending a funeral as opposed to a debate).
I think interviewers are more concerned that the applicant woke up and put some thought into what they chose to wear in anticipation of the interview, as opposed to how formal they dressed - i.e., shorts vs. a pair of khakis. The interviewers realize that the applicants come from all socio economic classes, so not all can afford or even have a suit and/or tie.
Both interviews were more casual than formal (the interviewer for Navy was washing his car while he interviewed my son). And all interviews were successful in terms of receiving a scholarship.
As respects questions, both were asked about leadership examples. So, your son should think of this area, especially if he had to make a decision that was not popular.