I have written a lot about this in the past and know quite a bit about PG years and the prep school scene. First of all, both Andover and Exeter are excellent schools. Kind of like Harvard and Yale or Williams and Amherst. Mentioning those examples as rivalry schools that are thought of on similar academic footing, with some nuances. That's it nuances. Neither are very easy to get into, and for PGs you are almost always recruited athletically (which is how you get int but still must have the academic chops) and usually have to agree to play 2 sports. There are other schools at or maybe just about the same level, but those two are almost always thought of as the top.
Next, a PG year for baseball is TOUGH. Baseball is played, in a normal world, in the spring. Why go to a PG to play baseball in the spring when you would go to college the next fall? If you have not been recruited before the prep season, it is unlikely to happen at an Ivy or other D1 school, or even the HA D3 schools who will already have done their recruiting. So the timing is tough. If your son is delaying college because he does not throw hard enough, I would caution you down this path. As mentioned above, he may never throw hard enough. Moreover, and please do not take this harshly, but the definition of insanity is doing the same thing and expecting different results. You do not say what year your son is, but if the Ivies were not interested in him, that tbh is unlikely to change by paying a lot of money to go do a PG year.
Feel free to ask any specific questions, happy to help