I have been in a similar spot. It might not be underachievement. For us, my child had to try so hard during the day just to behave in an environment where no learning was happening. Then to have to come home and do more of the same was the final straw. It eventually went from whining about the dumb homework to all-out crying and refusing to do it.
This is when we approached the teacher, and from there, we eventually skipped a grade. I guess I should have gone to the teacher earlier.
I think Howdy makes a very interesting point. Recently, I've been reading a lot of books regarding habits/change/etc. There's a consistent point in them that had never occurred to me before, which is that the longer a person has had to use their self control/willpower that day, the less able they are to do so toward the end of the day. For us, this helps explain some behaviors.
Have you tried telling him he can wait and do it in the morning if he likes? If it's an energy thing (as in, "I cannot do one more worthless thing today"), he might be more able to get it done quickly in the morning before school.
