Most PGs do an extra year because of athletics, so that will be hard to get away from, i.e. at most schools the soccer piece will be quite important.
I know prep school soccer well. Berkshire is a powerhouse, and they take several PGs as well as seniors. Not every Berkshire player goes on to play in college, but most do. The level of play is quite high. In the time I’ve been paying attention to prep soccer, they’ve had 2 boys become pro, Jack Harrison (currently at Leeds United) and Jacob Shaffleberg, at Toronto.
You say your son doesn’t want D1, but would he be interested in D3? If yes, then going to a boarding school with an emphasis on soccer, like Berkshire, would make sense.
South Kent is another soccer powerhouse. It definitely is worth investigating. They play high level soccer, for sure. The coach, Owen Finberg, is beloved, and he looks out for his players.
Other boys’ schools with higher admit rates are the Salisbury School and Trinity Pawling. Both have weak soccer teams. Avon’s isn’t all that strong either btw, but is stronger than Salisbury’s and TP’s. That said, you can be a strong player on a weak team and do just fine. Ousseni Bouda played for Millbrook, a very weak team at a small school and he’s at Stanford, having won Gatorade Player of the Year, and also Pac-12 Freshman of the Year. So, just b/c a school as a weak team (which is probably due to the fact that they don’t recruit) doesn’t mean it shouldn’t be in the running.
Depending on how your son wants soccer in his life, other options could be The Northwood School or High Mowing in Vermont. Those are the residential academies of Black Rock FC, and the level of soccer is very high (they play MLS youth teams, eg). If your son is wanting to play in college this might make sense.
I know nothing about the music programs of these schools.
I probably should have started with this: what is your son’s goal for the PG year? Does he want to play soccer in college? (As a rising senior, he should be in the middle of recruiting, if he’s interested in playing in college; also he/you should know what level he is at – is he a D1 level player? Something else? )
ETA: with his grades, Berkshire is probably out, unless he’s a soccer star that they really want. The soccer piece is what’s going to be the difference maker in admissions, fyi, so you probably should make that the driver of the process.