Many schools PG year is really only for recruited athletes, but you could call around! (And if your kid is a great athlete, this could help).
FWIW I think CH-CH Admissions | Day & Boarding Private School | Boston Area is pretty much rolling admissions, not a big sports school, and offers PG years.
I do question putting kids through admisisons process, 2x, that is a lot and kids mature quickly at this age. (Note, obviously you know your kid best, they sure might not be ready, but may be in 6 months ?)