I was always told you take it after pre-calculus. My son took it at the end of sophomore year. There is no Calculus on the test & students usually find it easier to take right after they finish that class.
One thing to keep in mind the Subject tests are offered in the fall. Why not take it Sept/Oct of his 9th grade year. As for taking many at once. Why? That just makes them more exhausting and more to study for. I would think it would be to your son's advantage to just take the one. And then take other later when he is ready.
