As mentioned above, all colleges know the New England boarding schools and understand the grading rubrics used at each, so let those concerns go. You also need to understand that the BS student pools are cherry-picked and comprise the cream of the crop from a global applicant population. It’s a highly curated group of achievers but, by definition, 50% of those “perfect” kids will find themselves in the bottom half of the class right out of the gate and at various times throughout their BS journeys. So, the question you need to ask if you are considering boarding schools is not how colleges evaluate grades but how resilient is your son? How does he deal with disappointment and what he perceives as failure? How will he deal with not always being on top? How have you modeled handling setbacks and what has he internalized from that?
Resilience is a primary trait boarding schools are looking for. For the most part, the BS pools comprise kids who are challenged rather than damaged by the many small “failures” they will encounter on their way to graduation. Somehow, the schools seem to know how to filter for this grit. There aren’t a lot of snowflakes in the BS communities, and those who enter at the more fragile end do seem to blossom rather than wilt in the process. Anyone concerned about BS damaging self-confidence might not be ready for or a good candidate for this option. These schools build confidence in students, but they start with kids who have already shown strong tendencies toward resilience.
If you search the archives here, you will find that grade deflation and how to handle not being at the very top of the class have been covered extensively, such as in this thread discussing what it’s like to be in the bottom 20% of the class. There have been many other relevant discussions on the college side, such as What Straight A Students Get Wrong.
The students who do best at BS (and in life) are those who were taught at home how to deal with setbacks and how to properly frame “failure” and whose high schools stretch them to take risks. Rather than looking for a BS that doesn’t deflate grades (preserves your status quo and insulates your son from the very important life lesson that he will not always be the best), you might ask each BS you consider how they help their students deal with the very paradigm they’ve set up. I’ve posted a few times how Choate attempts to deal with students’ fear of failure. They don’t always get it right, but they recognize the need to address the common anxiety that your son feels about his academic performance.
The bottom line is that you need to honestly determine what you and your son are looking to gain from boarding school that he can’t get from his local options. I always pound the drum that boarding school is about a stellar high school education, not college results. When the student is well-educated and resilient, college will take care of itself.