“wasn’t told his resume was too long”
Unless resume feedback was an explicit part of the program, I would never expect anyone but a career counselor to tell you that your resume is too long. That isn’t their job.
“DS did do a 1-page resume for his English class, and it was cramped and still left out many relevant items.”
Relevant to whom? I just don’t see many high schoolers out there with more work and activity experience that colleges and employers actually care about than will fit on one page. You are either including things that don’t matter or the descriptions are way too long. Most summer jobs and leadership positions can be boiled down into one line.
I also see a lot of HS resumes with all kinds of details about their HS coursework. This doesn’t make any sense when colleges are going to get your transcript. Wasted space.