Trying to take SAT/ACT tests in the spring or summer is very difficult, because baseball is played on most Saturdays. Is he going to skip a game? For Ivies and other high-academics you will also have to figure out a time to take SAT subject tests, that eats up another Saturday test date. The fall and winter are much easier to schedule (unless your son is playing football). For that matter, if you play in fall baseball tournaments/showcases, that eats yet more Saturdays. So, there are really only two or three test dates that are ideal for baseball players. For Ivies and similar, you really need an SAT score to tell them by late fall of junior year.
PSAT is something of a predictor for SAT, and can tell coaches (and families) what is likely to happen on the SAT, but in the end they have to take an SAT/ACT to the admissions office, so that's what they really want to see. Also, the PSAT in fall of junior year is the qualifier for the National Merit Scholarship competition, so should not be skipped.
Finally, many people do significantly better on SAT than on ACT, or vice versa, so depending on what you're aiming for (this applies to everyone, not just high academic types), it's a good idea to plan to do both. That's two different Saturdays.