Basic financial argument (assuming that a year repeated isn't going to change academic outcomes), 1 more year in the work force at 20/21 is going to add up to a lot more than is going to be gained from getting to play an extra year of HS Varsity (or the expected value from any playing beyond that). And that's before whatever you might be out of pocket for the extra year in private school tuition, etc.
Basic emotional/maturity argument, if your kid's capable of doing 9th grade work at a high enough level (for Georgia, let's say enough to put him on a path to Hope/Zell Miller scholarships), having him repeat a grade isn't going to help him, and might very well hurt. I've taught plenty of kids who weren't ready to go to 9th grade, and they don't want to repeat, I can't imagine holding back one who was actually capable of doing the work, especially if any of his peers was likely to find out/give him a hard time about it in HS. If there were actual maturity issues academically/emotionally they should have been identified before MS was over, and this decision could have been made at a less critical academic development point.
Yes, people do this all the time for sports. People do things for dumb reasons all the time. You're only likely to get success stories for this path from the people who will see this thread, because the people who held their kids back and had problems probably also ended up with a kid who didn't have his sports future pan out as expected, so they aren't going to be posting here.
My parenting background, FWIW. Oldest son will turn 18 next week, heads to college in August. Enrolled him in K a year earlier than necessary given the July birthday, based largely on his already being taller than everyone else, having spent most of his life in daycare, and his seeming to be more than ready academically/emotionally (relative to his K peers, anyway). Played his HS career with classmates who mostly ranged from 6 months to 2 years older than him. Probably would have had a slightly better HS baseball experience if he'd had another year before becoming a freshman, but hasn't stopped him from getting to play college ball at a D3 high academic which is very likely where he'd have ended up even with a later HS start. The later start wouldn't have made a positive impact academically, since he's always excelled academically, maybe some slight chance that he'd have been worse off (probably not, though) from hanging with a different crowd had he held back a year since he ended up with a completely different set of school relationships than baseball relationships with the year difference between his baseball age and his school class.