Re: when to take the tests. As soon as he has a shot at the score needed - the earlier the better. There is no penalty to taking multiple tests. The earliest ACT test, I believe, is September; that gives him the summer to study. Waiting until spring could mean a testing logjam if he needs to take the ACT/SAT again (a very normal occurance), plus multiple AP tests, plus HS baseball.
There are 56 kids per year getting into the IVIes for baseball; the competition for one of those spots is as hard as competition for acceptance for a regular student.
I guess I'm saying its pedal to the metal on multiple fronts - academically, baseball - because if your not doing it, others are. Moreover, I understand that the NESAC conference is the same - only in D3.
Re: positions to showcase in. Any position which gets him on the field. There are no reasons to not play (except injury).
Re: PSATS. I've always viewed it as more advisory to families rather than influencing colleges; it puts down a marker for the kid but it can't get a kid into a college.
One thing the drive to these HAs does is give an opportunity to mold a HS kid's discipline and time management.