I considered a ‘grade adjustment’ for my daughter. She started K at 4 and that was a mistake. I thought she’d just repeat K but they wouldn’t let her because academically she was fine and at the time she was socially and physically on par with the others (who were all small and immature at age 4-5). And then suddenly she wasn’t. Academically she was always fine, but I think she would have been a superstar if she’d been with the class behind her, with kids her age. Also many of the other kids weren’t just 6 months older than her but had been redshirted for K and were 16-20 months older than her. And bigger.
We also moved to California when she was 13 and starting high school. I would have loved to have had her repeat 8th grade but that would have meant starting at a different school and then changing to a high school a year later.
Her friends were always the other ‘youngest’ kids or the kids in the year behind her (who were often older than her). Socially, that’s where she needed to be. She sometimes was kept out of things because she wasn’t old enough - girl scout camp required her to be 7 and she wasn’t. R rated movies required her to be 17, and she wasn’t. It was easier to hang with the other kids who also weren’t 17.
She was a recruited athlete to a Div 2 school that was a perfect fit for her academically, athletically, and (for me) financially. She was still growing when she went to college. Those D1 girls are bigger. Most athletes aren’t just D1 or D3 and could play on teams at any level; my daughter was recruited by all levels, not the top D1 teams but at lower ranked team because she had good stick skills. My daughter’s D2 team could (and did) beat a lot of D1 teams, but there were D3 teams that could (and did) easily beat her D2 team. There can be a D3 track star who can beat a D1 runner, but the D3 team as a whole is not competitive against the D1 team.
I don’t think it is a stupid idea to repeat a year. Make sure you understand the rules. If you homeschool and it is considered 9th grade, he may use up a year of eligibility for high school athletics. Many states only allow 4 years of playing after 8th grade and count from the first time a student completes 8th grade. You don’t want him ineligible his senior year.
The issue with doing a post grad year at a prep school is that his entire high school ‘show’ years he’s playing as a little guy. My daughter was a good high school player, but she was small. Size does matter. If she’d had another year of playing, another year of growing, she might have been recruited to a stronger D1 team. Her teammates who were bigger were.
However, it all worked out.