Honestly, I think it’s far too soon as a high school student to start thinking about triple majoring in college. Being interested in multiple subject, yes, that’s great.
He’ll go to college, find he likes some things, loves others and maybe go deeper in multiple areas of interest or just focus on one. In any case, it’s rare to triple major without spending more than 4 years. Many colleges will also not allow it ( for good reason).
I was a person who had many interests and wove them together. There is no need to have a triple major on any resume. He could demonstrate his interests via the courses he takes and the ultimate degree he receives.