I'm not sure what the admissions calendar looks like but as we roll into December, but if you're considering doing a PG year next year (meaning your son is a HS Senior right now), then you need to start the process right now.
As far as the decision to pursue a high academic prep or not for baseball reasons, in my opinion, it's not about what he does during the prep year. It's about "resetting" the recruiting clock and making this upcoming season, spring of 2021, effectively his junior year all over again from a baseball perspective. You can let schools know that you intend to PG and re-classify, so they will essentially be looking at a player with another year of high school left, like a junior would have. By the time he plays his actual prep baseball season, he'll likely have already been admitted to whatever school he will end up attending.
In regards to attending an Andover/Exeter type school (there are many EXCELLENT preps out there in addition to those two), I would recommend it from a life experience perspective. The schools are amazing places, and the opportunities there are incredible.