Based on my 2017's experience, I would say HF is more about the academic record you present on the application, which then goes to the coaches who are working the camp. Certainly your 2019's GPA would attract some attention, and if he throws well he may get some value out of it. But from what I saw at the June session in Sacramento, the vast majority of schools are academic D3's and they do not really start recruiting until a kid's Senior year. I think they figure if your son is really good going to HF as an incoming Junior, he would likely wind up at a high academic D1/Ivy school and they won't have a chance at him. There are exceptions of course, so if your son has no desire to aim for D1 and wants to start making an impression on the smaller schools, HF is ideal. But the coaches he does impress are going to want to see his transcripts and test scores as soon as they are available.
Regarding PSAT/SAT, my son did really well on both, so I do think it is a good indicator of potential. For reference, my 2017 took the SAT in October of Junior year, then retook for an even higher score in January (with test prep in between) and the ACT in April so he had both test scores before attending HF June after Junior year. His GPA is only average, so the test scores definitely got him on the HF coaches radar. While I do not think HF screens out players based on their academic record, the coaches at these schools are very focused on making sure a kid has a chance to get into their school before they spend any effort recruiting a player.