I would ask by school. At some, they may feel they have enough fluidity between grades that being the age of the kids in the year below him may not matter as much. (And that may be the way to frame it in asking - this as a 10th grader or repeating 9th.)
I see a few on your list where the students almost always lean toward older. He would be younger than the freshmen at these.
Mine was on the younger end, and he had turned 14 prior to 9th.