Lots of good advice. I would add that if your son wants to attend a school like Caltech, he should not lighten his course load for 11th and 12th. Colleges want increasing course rigor, not "slacking" as a junior/senior. Perhaps slacking isn't the right term, but DD17 wanted to lighten the load for 12th, but was convinced to do otherwise. She wanted easier science and math courses. Finally decided on AP Physics C for 12th and Linear Algebra (so not an AP math).
Are there other areas that he can explore instead of blowing through the math and science? Could be academics, music or athletics. Top colleges see plenty of applications from kids who took lots of advanced math and science courses, with math/science ECs. Both my older two take/took two languages through AP level, eldest was active in band and both play/played sports (varsity & travel). Not saying that is "enough" to get a kid into top schools, but he will be competing against kids that have done a lot outside of math/science.
