Thanks for sharing, I'm glad your son has found his groove.
Both of my kids are studying for an exam to pass out of a grade level of math next year. I understand math, I understand the need for it, in certain circumstances anyway. Fuck, I was in engineering and manufacturing for 25+ years, I get it.
But for the most part, it's a complete waste of their time, other than the fact that they enjoy taking tests and like the challenge of moving ahead of their peers.
The vast majority of things "taught" in public schools here in Murica today are either incomplete, just plain wrong or the kids don't understand why they need it. They don't need it, the cash register does all the work printing the receipt for what the customers buy at the discount stores & chain restaurants with their plastic "money."