It's so much easier to think about this on the weekend, when no one else is awake and we're not actively trying to do school.

ColinsMum: Yes, his last year of school was very difficult, and I'm sure he hasn't recovered fully. He hates sheets of problems, and while I could try it, I'm pretty sure he'd look at me like I was nuts for even suggesting it. But yes, we are definitely going slowly right now, and I'm not concerned about that. I'm just worried about math grinding to a complete halt because "it's too hard."
I would expect that the memory issues were most likely due to the anger issue. Does he forget things he chose to do?
DS has a great memory for lots of stuff. Minecraft (and other games), Scratch, survival techniques, and so on. But they're all things he chose to learn and is passionate about. He is not passionate about fractions. (Does Vi Hart make videos on request?)
Cammom: We are using some tasty carrots, too, in getting DS to do math. We have a Singapore workbook that's just below his level. He gets some game time for each page he finishes, plus 10c towards buying a game he really wants. He's not exactly ripping through the book, but he is making progress. This is probably the most tedious math that's available to him, so it's completely up to him if and when he does it. Allowing him to work out of order was key, too, so he feels he has some control over what he does.
And yes, we do our math in very small chunks. A typical LoF chapter these days is 2-3 pages of text, with half a dozen problems. He can stop after one chapter. And while I would love to finish a chapter a day, it's been more like a couple of chapters a week recently.
HK: Perfectionism is a drag. I will think about making my example problems difficult. And getting lots of math problems from other sources. Here's what threw him off a week or so ago: If presented with the problem
1/10 x 5 1/2, he knows what to do. But the problem was stated:
What is one-tenth of 5 1/2? Wild accusations of "they never taught me how to do that!" follow. *sigh*
Then I ask questions-- or make errors ON PURPOSE... and have her 'check' my work.
Then I let her show me one.
When you do this (let her show you how to solve a problem), do you pick the problem, or does she?
Val:
Do you think your son could be shutting down because he feels intimidated?
Hmm...intimidated. Perhaps, although the stuff we're working on isn't that challenging, so I wouldn't think it would intimidate him. It does require at least a little practice, though, which is tough to implement. Hence the Singapore math and heaps of rewards for doing it. And it does sometimes require a little bit of thinking about what strategy to use, something I haven't seen Singapore addressing (yet...but we just have the workbooks, not CWP).
phey: I am super nervous about just "dropping" math, even temporarily. Can I really do this?

I totally agree about reducing the amount of busywork, though. How to find the line that allows him to practice what he needs to practice, while not straying into drill-and-kill territory.... I like the idea of trying some CWP now and then. I suppose the best strategy is to start low (a grade below where he's at?) and ramp up over time. And explain incessantly that I don't expect him to get the answer immediately, or get it correct the first time.