From a cost-reduction perspective, keep in mind that AoPS books are very clearly written and easy to follow, and its on-line community is extremely active and free to join. For a kid who is a natural autodidact anyway, he may do great with the books and on-line access, without needing the actual (quite expensive) on-line courses to keep him moving along. If he has questions, other kids and monitors on the AoPS forum could probably help a ton, even if he doesn't have direct access to a teacher via a specific course. Or he could do one class as a formal course (I'd tend towards the coding, personally), build some comfort and a network, and then do others on his own.
I would expect by now it should even be possible to find some AoPS books in used form.
Awesome you are doing this, eco! I think you and your DS will both benefit greatly. Good for you for being brave enough to try a new path. (And for what it's worth, I suspect there is a vast range of emerging careers out there based on electronic music composition. Though my old-school mind can't conceive of them, I think my DS is busy inventing a few. The intersection with coding could be particularly cool.)
