RichinPitt brings up a good point, which I didn’t know, so if you have to take it between soph and sr, you might as well wait to take it in the fall of jr year so you can use any prep your son does for PSAT for the SAT.
Be careful of taking subject tests in 9th grade, you’ll be competing with a 11th grade version of yourself! meaning kids taking the math 2 will have taken stats or m/v calculus, AIME tests and will breeze through the test, probably taking it in half the time and getting nothing wrong, you don’t want to be curved against them. They take it jr year along with the other subject tests in one sitting.