Well, I specified rigorous prep school in my OP because the school is very well known, and very well known by AOs at any school he would be applying to. So luckily there isn’t a question of a suspiciously easy or superficial class.
I have asked the question of whether it would impact my sons college admissions chances not to take the test of the college advising office. Happy to report back with their answer in case anyone else happens to be in this position.