Math Classes for Kids

To supplement would mean to take alongside his current course … the programs you mention are all for credit and would need to be approved by your sons HS. He’d only get into a class that he’s done the prerequisites for. I think what you may be looking for is something more like Khan Academy, which is excellent. 

One of our kids took a math course through Apex and I thought it was terrible.

We have had two kids take CS courses through UC Scout and thought they were meh.

Two kids took various classes through CTY (Center for Talented Youth at Johns Hopkins) and thought they were good to excellent, depending on course.

All three of our kids have taken math classes through AoPS (Art of Problem Solving) and thought they were amazing. They are very hard though and get harder the higher the level, but I thought they gave our kids an amazing grounding in whatever class they were in. At one point our district stopped giving credit for the classes but that wasn’t why we had the kids take them so we didn’t care, but if you do care it is something to be aware of.

My daughter took geometry on SVHS. It is basically a collection of YouTube videos of various instructors explaining different concepts and then problems and tests on those concepts. It is not engaging but maybe fine, depending on the kid’s motivation. She is also doing AP French on UC Scout now. The materials are OK but the teacher does not give real feedback on the work. It seems like she grades it without reading it. My daughter is having to supplement with a tutor. I don’t know if math is similar. None of these are a good idea for a kid who needs to be drawn in and engaged. They are better for kids who are determined to slog through to achieve a  particular goal of their own, 

We did BYU. There appears to be two levels (presumably college prep which is "Teacher Led" and regular math. Teacher-led costs more). The Algebra 2 course was not easy and the second semester included some basic coordinate geometry, probability, and statistics.  One must be motivated to get through it, but my son soldiered on and did well (not his favorite subjects). There are BYU teachers/professors available for tutoring help, but mostly Kahn Academy and us (parents) provided some support. Bonus - the ability to re-take the chapter quizzes helped boost his grade. I recommend taking the mid term and final at a locally-available proctor (community college, Sylvan, etc). The online proctors at BYU were not good.

My son's independent private school in Kansas City offers an online program as well as traditional in-person classes. I've known kids to do it over the summer and have heard reasonable things about the availability of the teachers, etc. Here is the information on taking single courses through the program. https://www.barstowglobalonline.org/academics#ind-courses  

I can recommend liveonlinemath.com. We used it for several years from pre algebra through algebra 2. They also have pre calculus. They have a proprietary software program to deliver the content, online live classes with good teachers that the owner (who was the only teacher initially) has curated, and they offer inexpensive tutoring if your child needs additional help. They offer live classes as well as a recorded option that’s cheaper. Solid classes in terms of content.