Decades ago, it was typical for students who needed to take Achievement Tests (which are now called SAT Subject Tests) to take three in one day at the end of junior year, corresponding to courses being completed then.
Agree with both of @melvin’s comments above. Our GC recommended taking in June as opposed to May to avoid being too close to APs. They also had a list of how the different classes lined up with the subject matter on SATs - especially which students should take Math 1 vs Math 2. It was pretty typical for S and his peers to take 2 or 3 in one day. S’s colleges only recommended/required 2 tests so that was what he did.
Thanks everyone for the very good advice. I completely missed that there was a test in June as well, so he will likely split up the tests and do one in May and the other two in June. I think Spanish in May makes sense, but we have time to figure that out.
Our GC had just the opposite rec. The ST only takes about 45 minutes to complete so it was a good warm up for the AP. (In my kid’s year, the ST was the week prior to AP.). Further, the June test could be used for non-AP-connected STs.
Obviously, the moral of the story is ‘whatever works best for your kid’.
Posh. Back in the true dark ages, we took the SAT in the am and the Achievement Tests in the pm. One-and-done! 
My youngest took the subject tests in May and liked being fresh off the APs so no real additional studying was needed. My oldest took a look at the physics SAT and decided it had been too long since he’d done all those topics so he took the test in June so he could review. (He was taking Physics C at the time.) Both kids also had Regent’s exams in June in at least some of the same subjects as their SAT tests so that helped as well. My younger son got his May scores in time to decide to retake the subject tests because of the (relatively) low math score. The original plan had been to retake the regular SAT in June - he ended up pushing that off to the fall.