@Houston1021, usually a little before April 1, I think. I just went back though the correspondence trail to when my D applied two years ago, and that year they sent out an email on Saturday, March 21, announcing that they would be sending out decisions the next day. A Sunday, kind of odd.
My D, homeschool 2015 graduate, took French III and IV from Keystone. We were extremely impressed with the courses, a lot of teacher-evaluated lessons, as opposed to fully-computerized ones, and an extremely reasonable price tag.
You should run a Net Price Calculator on the school website to get an idea of what your need-based aid would be if you were to apply for it. And as @Madison85 says, you need to find out how much your parents will pay for subsequent years, and see if this is a workable option for four years.
Congrats to your son on qualifying for AIME in sixth grade! Is he doing AoPS? Your reference to The Calculus Trap makes me think maybe so. I would have him take as much as possible from them before moving on; they are a tough act to follow. He sounds strong enough that their WOOT class could be of interest to him before too long.
My homeschooled daughter did all of her high school math with AoPS, and took their wonderful calculus course in ninth grade. After that she switched her focus from math to art for a couple of years, so the competition stuff wasn't a focus anymore, but finding interesting options for Calc II and III wasn't easy. Most were dreary and mechanical after AoPS, which was all proofs. Is there a university nearby where he could take dual-enrollment courses? There you could find more interesting approaches to Calc II and beyond on the more theoretical track. Is there a math circle in your area?
For distance learning, Stanford Online High School has MV, Linear Algebra, DIffEQ, even a couple of Analysis courses. And eIMACS has a wonderful series of mathematical logic classes.