That's a tough one - you can make him do his homework, but you you can't MAKE him care. Any kind of punishment or reward can get short-term results, but can't REALLY get to the heart of the matter. True motivation and passion comes from within. And human nature being what it is, sometimes rewards and punishments have the opposite effect from what you desire. (Like *having* to read, say, Huckleberry Finn for English class, rather than finding it at the bookstore and choosing to read it on his own.)
I read a quote recently (can't remember who said it) - something like "Education isn't so much about filling a cup as lighting a fire." Is there something he really is passionate about? When he goes to the planetarium show, or a museum, or the zoo, is there something that really gets him interested? Is there a way to feed into those strengths? Many people who are very focused and single-minded may be able to thrive as adults, because in the adult world, you *can* (for the most part) just focus on one thing and get your PhD in literature, research nuclear physics, bake wonderful desserts, build custom cabinets, play the guitar, whatever! But while they are kids, there is still an expectation to learn (and be somewhat interested) in many areas. And it can be difficult for some kids, sometimes even the brightest, most promising ones.
So see if you can figure out a few things he's really passionate about and make sure he has enough time to spend on that. Fill the house with books that interest him and maybe see if you can leverage his strengths into other areas. (Like doing math drills are boring, but if a sports buff is using math to figure out batting averages and other statistics, he can see the point of it and he'll have an incentive. I've read that many children in poor countries who have to work as street vendors become very savvy in arithmetic and math at a very young age (at least enough to do their transactions), even though they don't go to school. They've got a very strong incentive.
I want to share a personal story about my grandparents trying to motivate my father. My father was an engineer and was always reading books about physics, science, and engineering. It was something he enjoyed so much that I never understood why he hadn't ever gotten an advanced degree or persued research. I mean, he spent most of his working years in decent engineering jobs, but... considering how bright he was, I think he was a bit of an under-achiever. Many years later, I learned from my mom that his parents (my grandparents) always had *very* high expectations for him, and they actively managed (or "nagged", like you said) to make sure he did all his homework, got good grades, etc.
He never liked an argument, never much of a rebel, so he just did the work. And he was valedictorian at his school, and I'm sure my grandparents thought they did an excellent job of "motivating" him. But it was all external. And went he went away to college, he really struggled. He had always been working hard for someone else, but not whole-heartedly. I think he just got burned out too young, and my grandparents never really saw that - they saw the valedictorian and college grad and just assumed he was sucessful. So... I guess this is a very long-winded way of saying be careful about how you try to motivate him.