My 9th grader is a true freshman (14, end of May birthday). He attends a smaller school in the LA City Section, so he was able to make the varsity team. However, we have heard many stories of good ball players who cannot make their freshman team at private schools primarily because they are at least a year younger and thus smaller than most of the other boys trying out. He played on a very strong travel team last summer, class of 2021 team. I believe only two of the players on that team qualified to play 13u last summer, as most had been held back at some point. As one of the youngest and smallest, he had to step up his game. I think it was good for him. He has been adding weight, so he'll likely catch up to the other 2021s in size soon. A friends son went to a D2 school, they said when they were recruiting him that they liked the fact that he was only 17 as a hs senior, as he still had time to grow and get stronger.
We have no regrets not holding our son back, but depending on where your son goes to hs, you should be aware that in some areas the practice of repeating 8th grade solely for sports purposes might be more common than you think.