A parent wrote in about an autistic child who was having trouble with math and, as a consequence, refusing to do homework. Was this rebellious behavior, or was it a learning issue that should be addressed?
You've given a good description of the problem (as far as one can tell from so far away), but it's still impossible for any of us to be as knowledgeable about the situation as you are. Obviously. And in my case, since I've never been a parent, it's likely that whatever I say may be unrealistic. There. I've provided some grains of salt to be taken with my comments.
Sounds like your son is running into a specific learning problem (whether it amounts to a "learning disability," I don't know). I had the same experience with math as I moved from middle to high school. (Not to mention the meltdowns I had in elementary school, come to think of it!)
From my own experience I can report that when one is used to having learning happen more or less unconsciously (perhaps as unconsciously as NT kids "pick up on" social stuff?), to run head-first into a problem where the learning simply does not happen...that is extraordinarily icky and frustrating in the way it feels. Suddenly, you lose faith in yourself. Because suddenly you realize (on some level, though you can't put it into words) that you don't know why the learning does happen when it does. It just seemed so normal that you've taken it for granted, but now it "doesn't work" anymore. Or not for this one problem area. And it's like your brain is broken. If your child has the perfectionist streak so common in auties, he'll find the combination of pressure to perform and inability to "think" (in the required manner) especially frustrating.
I got no significant help when I was in this situation. The teachers sort of colluded tacitly to let me slip by. These days, parents of autie kids seem to be turning to computer-based learning aids. Parents on the OASIS maelstrom list (see http://www.consultmac.com/AS/ for info) have posted about the various math-aids they've found that worked for their child's particular way of learning.
One element that is necessary, whatever method one is using,
is repetition. Repetition in the manner that is not helpful
for your child may only compound the problem, so it is
necessary to find the method that suits his learning style
before the repetition begins.