AoPS's style doesn't seem to match everyone's taste. Two other often mentioned alternatives floating around are:
* Elements of Mathematics
https://www.elementsofmathematics.com/* eimacs:
https://www.eimacs.comA few other things stuck out at me:
Using the AoPS textbooks is very different than trying out problems via Alcumus. Assuming you only did that, you still may want to give the books a try. The books are inquiry based but formal. You usually start with some problems and then develop the structure after trying to figure them out. (Ditto for the online classes)
Also, is he able to actually solve the AoPS challenge problems? I'd want to know that before writing them off as not interesting.
And then the topics he like are not particularly standard k-12 curriculum. (Many do show up in contest math btw.)
So honestly a good math circle would be very likely to cover some of them and would not just be playing with same ideas. Have you looked at the Julia Robinson Festival problem sets?
In the end, there is no one right way to learn. So trying some of the alternatives seems like a solid plan.
