I am new here. My DD10 and DD6 are in CA public school so I can answer your question.

My DD10 is happy self-studying Algebra 1 at home. In Grade 4, she self taught prealgebra in one summer and can answer 90% the AoPS exercises correctly. I have not enrolled her to any of the AoPS online courses so far due to our inflexible schedule.

Her school uses the California Math Expressions Common Core textbook.
https://www.thriftbooks.com/w/math-...oftcover-grade-2/9283814/#isbn=054782452

Here is the CA Common Core math resources page:
https://www.cde.ca.gov/re/cc/mathresources.asp

These common core textbooks teach different math procedures from those in traditional math, in a way that they emphasize a lot of *inexplicable* intermediate steps instead of direct mental calculation. DD10 does fine with math at school as she adapts herself well between the two systems.

However, my DD6 is a bit struggling with the school math. She is an over achiever at her age as she can do long multiplication and division with remainder already. She scored 100% in her exam, but as she calculates results mentally she is considered not fully grasping the math concepts at school, while the school teaches addition up to 10.

Many CA private schools adopt Common Core as well, but they use different textbooks and often are one year ahead of math curriculum of public schools.

I won't recommend anyone to enroll an online course to integrate into the Common Core system. If your kids are strongly interested in math, that's another story. You mentioned math comes easy to your DS. If he is used to mental calculation, he might need some time to adjust. Honestly I do not quite understand the math procedure of Common Core myself as a STEM PhD, and I couldn't care less. No matter what core it is, math is still math.