I'll echo spaghetti's suggestion to check your district's website to see if they have any information on course alignment, and would also suggest asking your ds' counselor for their recommendation. Our high school counselor suggests taking Math 2 after pre-calc, and taking Chem and Biology after AP Chem and AP Biology. Our counselor also recommends not taking more than 2 unless you've verified that the school you're applying for requires more than 2, and also recommends that you make your choices re which tests to take based on what the student wants to choose as a college major. I'm not familiar other than vaguely with all the ins and outs of choosing to take the tests more than once and using the Score Report option to only share your top scores, but fwiw our ds' counselor doesn't recommend purposely taking the tests and then repeating them to get a better score - her take on it is it typically doesn't buy much in terms of actual score and it's something that is sometimes seen as a negative by admissions. If your son likes standardized tests, I'm guessing he also tests well, and really wouldn't need to worry about retesting to get a better score, so I'd just make a plan based on when he'd completed the courses that feed into each subject test.
The one timing thing you haven't mentioned - spring of junior year can get a bit test-crazy - it's not a bad idea at all to get some of the subject tests over and done with before then, if he's going to be taking a bunch of AP tests and also SAT+ACT.
Best wishes,
polarbear
