Some good thoughts so far, so I'll just add one more to the mix. What if the problem isn't ADHD? What if doing the work itself is so unpleasant to him that he'd rather face negative consequences (or at least take his chances, because sometimes he gets away without them)?
This is something I've frequently experienced (even as an adult), where the task is mundane, I see little or no value in it, nor receive any sense of joy/accomplishment, but it's taking up a lot of my time and effort. One coping mechanism is to escape into daydreaming or deep thought, which works great for tasks that don't involve a lot of brain power, like washing dishes or cutting grass. But when the task also requires a significant amount of concentration, there's no such escape.
In these situations, a very effective strategy is to play some music. It paradoxically provides a distraction, which helps me focus on the task at hand. In my idle brain moments I'm not thinking about how much I hate the task at hand, I'm listening to the music instead. It also helps to alter my mood from one of negativity.
These days I'd also drink coffee, or if working on something at home, nurse a beer or two through the effort. I don't recommend either of those to a 13yo boy, but the idea is to provide some source of pleasure to offset the sense of dread at doing an unpleasant task.
