I have a son who did a PG year and went high academic. His situation was more about a significant position change which was creating a whole new recruiting environment for him. Like yours, he was young in his class, though maturity, physical or otherwise, wasn't really an issue.
Twoboys post above is a very good one, I'd agree. Of course, there are unique situations that may create an exception depending on your situation. A PG year will likely make him bigger and stronger, whether that creates additional opportunity or not is unclear. It happens, but I wouldn't say it's the norm, unless there are circumstances like in our case where a position change junior-senior summer changed the landscape. In my view, chasing college scholarship by spending money on prep school is most often a losing proposition.
All that said, a PG year at a very high academic prep was a wonderful experience for my son. He absolutely loved it, made lifelong friends and contacts and I know he would do it again in a second. He had an excellent baseball experience at the school as well.