No question about it, being held back will have him physically bigger and mentally more mature than if he wasn't. No guarantee he will be a better baseball player though. If main objective is to try to get as much college scholarship opportunities or pro looks then the gamble of holding back may pay off. Can also backfire though. Assuming he gets a scholly, redshirts freshman year, has a decent 3 years playing D1 ball, enters the draft as a 23 y/o Jr, might not be too desirable in draft eval. 2 - 4 years in minors minimum, now a 25-28 y/o trying to reach the show. I know of 3 cases where the above is happening, and I'm sure there are cases where it turned out great (just I'm not aware of them). If it makes sense to be held back excluding athletics/baseball then might be appealing. There is much more to being a top baseball player in higher levels than just being physically bigger in high school. Trust the process.