Ask HN: Math online course recommendation for a High schooler

My son's a 9th grader and has completed a couple of online Math courses on Coursera, in particular:

1) Introduction to Mathematical Thinking (https://www.coursera.org/learn/mathematical-thinking)

2) Introduction to Logic (https://www.coursera.org/learn/logic-introduction)

In his school he is currently doing Pre-Calc and will be taking AP Calc in his 10th grade.

He's looking for some online Math courses that he should take to help broaden his knowledge.

A couple I've asked him to look at are:

1) Game Theory (https://www.coursera.org/learn/game-theory-1), and

2) Discrete Mathematics specialization (https://www.coursera.org/specializations/discrete-mathematics?)

Would appreciate any advice for online Math courses. Doesn't have to be Coursera. It'd be nice if the course offers free audit with a payment only for certificate, but is not mandatory.