Just to echo a couple points made above-
The voice teacher will be a very important factor in the decision, but the overall strength of the faculty is also something to consider, as teachers sometimes retire, take sabbaticals, have health issues, or even leave for other jobs, and since undergrad is four years the risk of that happening is maybe more of a consideration than it would be in picking a two-year grad program. Of course after the program admissions come in, you’ll need to find out what studios at each school are an option- that can help make the decision.
The quality and number/availability of practice rooms turned out to be a bigger factor than I expected in my D’s decision. What I hadn’t really considered (and she explained) is that VP students need to make big sounds sometimes (frequently) and the ability to do so without feeling like you’re performing through the door all the time is very helpful. Also, nice practice rooms you enjoy going to facilitate more practicing.
For my D the availability to pursue some other academic interests outside the music building was important (I realize it isn’t to all music students). The convenience factor of having those options nearby on the same campus became important because in her mind it increased the likelihood of success should she pursue that (which, by the way she is, so far, and enjoying).
Cohort I’d think about in terms of the spirit of the institution, as the specific personalities shift every year. What types of students do they attract? My D also picked a program with grad students, which has been fine, she still has plenty to do.
Cost of attendance, of course, especially if grad school figures in the plans.
We also sought some outside advice from whatever extended academic and professional networks we had some access to, which proved helpful in giving another viewpoint on the various programs from the other end, even if we didn’t follow that advice completely. I should add that in each case we had good reason to believe these folks knew what they were talking about.
I’m sure your S will be fine, although part of his conflicted thoughts I’m sure is that it can be hard for these kids to let go of admissions after putting all that effort into earning them, and picking just one. If my D could have cloned herself last April she probably would have.
Good luck!