When your son is a bit older, you could consider Awesome Math Camp.
https://www.awesomemath.org/summer-program/My eldest kept getting their brochures. We don't know who submitted her name, but we laughed about it as math is her weak point. It is expensive, but it would be a place to meet others like him.
Check out mathzoom.org as well. Expensive, but again, might have kids at his level. There are some free or low cost camps for high school kids, but he wouldn't qualify based on age for a while.
Are there any Math Circles near you? Look at mathcircles.org They might provide a challenge and there would be other math loving kids.
