We are in a similar situation in that there were not a lot of resources for our son when he was younger. Unfortunately, Beast Academy was not yet available. If we were doing it again, we would definitely use Beast Academy, then AoPS.

With the internet, it is quite possible for your son to learn all the math and find a community of learners, like him. Not ideal for many kids, definitely including ours, but it is better than them not learning at all.