First off, you're not doing anything wrong. You just happen to have a kid who is a learning machine. If it makes you feel any better, DS8 was teaching himself how to do the math from the pre-test I was giving him when we started.

As I recall, he wound up doing 2.5 or so years of Singapore Math that first year before I gave up and switched him to high school geometry just to keep him from hating math. (And I don't think he's nearly the challenge that your DS7 is!)
In short, it's not you or anything you're doing. Honest!

We did finally hit a point at which he wasn't just soaking things up a warp speed. That's no guarantee that your DS has one of those, but there's hope at least.

Is he unhappy? Do you think the perfectionism is being caused/exacerbated by the fact that the (really advanced) work is too easy for him? Or is that just how he is?
Other than the natural fear any mother would have at the thought of a very young child leaping way ahead--like college--is there any reason you can't let him leap way ahead? Slowing down by going wide doesn't work for all HG+ kids. He may just need more challenging work, and letting him chart a course may be the way you have to go.
Can you find some big projects for him that are more involved? Can he perform his own experiments, say? (Real ones!) How about creating his own computer program, maybe even something he could ultimately sell to a software company?
Don't kick yourself. The first year is ALWAYS a learning experience for the homeschooling parent. ALWAYS!