I’m in agreement with so much of the advice here, but I’d also offer a different perspective about visiting: an admitted students day (or big prospective students weekend) is not going to give him an accurate view of what these colleges are actually like on a day to day basis.
If his boarding school has a big festival weekend, or big annual sports weekend or some such - ask him to picture the impression that a person that is visiting his boarding school on that atypical exciting weekend would have—he’d probably recognize that dropping in on that Saturday certainly wouldn’t give an accurate perspective of what his regular day in, day out weekdays are like as a student at his school.
Yes, big weekends and being wooed are fun and exciting! But they’re not everyday life at those colleges.
Visit the colleges when it’s a normal day, when nothing special is happening, because that’s what a day in the life would look like for him. Have a meal at the dining hall, sit in on classes, spend time in the library, the student center, areas of his interest, check out a student group meeting…get a feel for what it’s really like to be a student there, rather than a guest that the admissions department is wooing.