Yesterday I spent about four hours of mortal combat fighting through about 2/3 of my son's homework.
This morning we spent 15 peaceful minutes doing the rest.
What was the difference? Well, maybe it was mostly the variability of his neurotransmitters -- today they were in better shape. But maybe my latest innovation helped.
I divided his homework into subsections. Correct completion (meaning he accepts my guidance and allows corrections) of each subsection earned him 5 minutes of computer game time (Backyard Baseball -- no educational value. This was valuable because he probably gets about 50 minutes hour every two weeks of computer time and it's usually educational software). After each section he could decide whether to continue his homework or to take a break and use up his game credits.
Tracking the five minute sums was also a useful 'real world' arithmetic experience for him.
We'll see how this method holds up. It's similar to what I do for myself when I face a large and daunting project. I break it down to portions, and get the "reward" of completing each step. In my case the rewards are more abstract, in my son's case they are more concrete.