This is a really hard question to answer, because no I don't think you made a mistake in placing your son up a grade. And, I think those that have given the advice to get bigger and stronger are absolutely correct.
However, I can think of three different local kids that my son played with who had summer birthdays. All three took a "homeschool year" between 8th and 9th grade, re-entered 9th grade a year later, and went from being the youngest in their class to the oldest. All three ended up D1 baseball commits.
But the landscape is changing. My son's class, the 2019 class, seems to be showing what the future will look like to some degree. I have seen so many of my son's 2019 friends, former summer teammates, and others from that 2019 class enter the portal recently. Many of them P5 guys, draftable guys, that had PG grades of 10, ranked in PG's top 200, and 4 year athletic scholarship money.
Adbono and others have pointed out why many younger, unproven guys are at risk. With the new rules there are predictable changes going forward:
1. Coaches are bringing in new guys, older guys, graduate guys, JuCo guys that can play and challenge for positions now.
2. Those additional 5 (27 to 32) scholarship spots will go mostly to secure transfer guys, not current walk-ons, and not 2020s that don't already have an NLI.
3. When the NCAA likely approves the one-time transfer exception in January 2021, the waters will become even more muddied. There will be no way for rising HS sophomore or even a rising senior to have any idea who will be competing against him for playing time his freshman year of baseball - two or four years later.
4. This situation will be especially difficult at D1 "baseball schools". (I don't say P5 because schools like Southern Miss, CS Fullerton, Coastal Carolina, etc. are better than many P5 programs). If the one-time transfer passes in January 2021(and it most likely will, COVID was the only thing stopping it this Spring), "baseball schools" will go back to scouting the summer leagues for proven college talent just like they did pre-2010.
So young or not, a HS kid that wants to play competitive D1 in the SEC, ACC, etc. needs to focus on velo. Theses are the top three things that get you scouted. Velo off the mound is #1, Exit Velo off the bat is #2, and Velo in the 60 time is #3. Those are the things that I would tell your son to work on, because even if he leads his league, region, state in all stats, it won't mean as much as those things.