It's sort of the other direction from the way you're thinking, but we're loving
Alcumus. It's problem-based, with instruction as an add-on, which is why I say it's in the other direction - but I feel strongly that RR is right and that mathematics is much more about learning to do it than about learning techniques. It does have generally very good explanations of how to do each problem which you can access if you get stuck, an assortment of video lessons (your DS would need earplugs/phones if using these in a classroom where others were doing other things), and accompanying books. (You can have it choose "focus topics", you can choose your own, or there is a "Follow Book" option which might be the best in a classroom context.)
I started a thread about it
here but didn't get reports of other people using it - which surprises me, given how many people use ALEKS/EPGY etc.; I think it's mathematically far better, and right up the street of many here. And no, they don't pay me :-)
