@uptick, welcome! Merit aid is such an opaque topic, is instrument and school specific, and may or may not be need-informed! Yikes! It can be hard to wade through, for sure! I would start by searching the Music Major forum for your son’s instrument and/or “scholarship” or “merit”. You could also start a new thread there asking more specifically about your S’s instrument or the programs he is looking at specifically.
As to trial lessons, as @glassharmonica and @MusakParent and others said above, I agree that they can be SO helpful. When @Proudpatriot says they can have " an equal chance of working for you as it has working against you," I agree only insofar as relates to being offered admission! And even then, I suggest that it is always best to study where you are wanted and valued, so in that way, perhaps trial lessons help even when they lead to a “no, thank you”. I vehemently believe that it is never a good idea to have a trial lesson with the idea of somehow getting a leg up when it comes to auditions. Trial lessons are an EXCELLENT way of getting to know a potential teacher and sussing out FIT for that most important, teacher-student relationship. Both student and teacher will, of course, form opinions about one another (that’s why they are so valuable). Some teachers prefer trial lessons occur after offers of admission, (to reduce the sheer numbers of the lessons), but many will give them before. I don’t think it much matters when you do them.
Vocal Performance auditions are never blind, and I think most colleges want to view their instrumental candidates, as well (for all the things that personality and visible technique can tell them).