Should your son attend HF at all? Only if the schools on his vetted list will be there. If the answer is "yes", then by all means your son can (and maybe, should) attend as a rising Junior even without SAT/ACT scores. And yes, the PSAT is a reasonable proxy for the SAT, though many kids improve their scores.
This may be obvious, but I'll add that attending any showcase or camp for purposes of college recruiting, not just HF, should be driven by fit, i.e., will the schools on your son's vetted list be in attendance. Or in the case of a PG showcase, your son will be rated and that information can be shared with his vetted list of schools even if they don't attend the PG event.
wrt HF, my boys did HF Jupiter in the Fall of their Junior years not HF Sac or NY as rising Juniors. IMHO, most schools at HF Summer sessions are very focused on rising Seniors. HF jupiter is typically in late October or early November and the focus shifts considerably to HS Juniors. Going again to HF the following Summer as a rising Senior allowed my guys to be seen twice by many coaches. It also allowed them to get a few more months of physical, emotional, and baseball maturity. And they both had ACT scores by then.