Sorry to hear about your son not getting in. Must be a huge letdown for him. Good luck to him as he determines his next steps.
I don’t post much, but I thought I’d just share something I learned from someone who recently read/scored applications for BYU. She told me that while grades/ACT are still a major consideration, BYU’s holistic approach puts a much higher emphasis than ever before on the essays. It’s less about checking all the boxes and more about demonstrating things like desire to attend BYU, resilience, unique perspectives, drive, lack of entitlement, etc.
She said that in just about every case where someone with a high ACT/GPA doesn’t get accepted, it is because there was a red flag with the essays or with the recommendations from the teacher (seminary or core subject). If, for instance, a teacher reports that a student had a bad attitude or seemed entitled or was always procrastinating or whatever, it could have a big hit on the kid’s chances.
Of course, I don’t know if any of these things applied for your son’s application, but if everything else was in order, that’s the likely cause.
As others have suggested, it might be worth a call to the admissions office—and maybe reapplying again in the future.
Good luck!