I could have written *so* much of your post. DS's grasp of conceptual math and problem solving is so far beyond mine, he leaves me in the dust daily. Yes, he has the answer while I am still trying to figure out the question. And yes, he is doing instantly in his head work that I am spending 20 minutes trying to lay out a system of equations and solve. I hear you, sister!
My suggestion is AoPS pre-algebra, which he sounds like he would be ready for. The book teaches absolutely everything covered in elementary school math, so any topics he doesn't know, they will explain. And do it well. If a lot of it is new, you just move more slowly.
AoPS is a fabulous way to learn. The explanations are clear and coherent. Everything is derived from first principles and seems to appeal hugely to my mathie's conceptual approach. (And I learned an embarrassingly large amount, even from pre-algebra. Especially about why you do things, not just how.)
More to the point, though, is that you buy the answer guide, too. Detailed, step-by-step solutions, even for the easiest stuff. So you can keep up.
And you don't just know the answer, you know how and why it's the answer. Often, with multiple different but valid ways of getting there. Great for you, as well as for the kid who knows the answer but can't explain why it's true.
(Be forewarned, though - DS likes to hide my answer guides and make me try to work out the problems myself first without peeking... He delights in "making Mom's brain explode with math!")
