<p>The harsh truth is that admissions at day high schools in the Boston area is fierce. (much fiercer, I think, than in your day, ExieMITAlum!) For nearly all of the schools, 9th is the admissions year, and the only slots in 10th are from those who drop out of a school - generally about 5% on average. I don’t think there is any reason to think that the OP’s son needs a maturing year - I suspect he is just a victim of the numbers.
Are there any schools at which he is WL that would be a better fit (in his eyes or yours)? You might have to lose a deposit, but a few spots may open up soon.</p>