Way too many Subject tests. No need for more than two or three. Georgetown is the only school I know of that recommends three; others, if they do recommend or require them, only ask for two. As you've noted, some schools have specific tests required (typically are STEM oriented schools/majors), so pay attention to those. Also, "recommended" means "required" unless you have some very special hook.
My older two took three Subject tests each. They took them at the end of a course or when they felt ready to take them. My eldest is a humanities kid, and after taking two humanities type tests, took one of the math Subject tests. My middle kid had taken two STEM type tests, and took French for the third. They based the third test off of the practice Subject tests - whichever one they could score well on, that is the one they took.
As for score choice, you can't just report your best scores. If you choose to report a particular test date, and you took multiple Subject tests on that date, you must report all tests for that date.
As for the French Subject test, I think he should wait until he is at 4th year level or beyond (or 3rd year if that is his junior year French course). How long has he taken French? It is fairly easy for a kid who can read the questions and has a feel for what looks or sounds right. But you may not get that from a couple of years of HS French. And remember that the French AP exam will fulfill the foreign language requirement at a lot more schools than the Subject test.
