He will dress differently on OV depending on what he is doing. For campus tour and meeting with adm then pants collared shirt are appropriate, the rest of the time he will be with players and team so shorts/casual shirt work just fine. He will likely sit in on some classes so dress like a college student.....well like some of them. No hat when with admissions and meeting coaches. with the team then wear one if he wants.
A humble but confident player is ideal. Ask lots of relevant questions of players on the team, don't be shy they will be more than happy to explain the inner workings of the team and coaches.
Interviews are great and should be done if possible. Do one at a school that might not be your highest priority first so he has some practice. We also role played with my son and had him ask us questions so he got comfortable with his delivery. He will resist it at first, but thank you afterwards.
Good luck, exciting times!