I was also somewhat above average throughout elementary and high school, except when I got to high school, I found myself increasingly struggling with "left-brain" subjects, especially math (this was partially because of Mr. Graham, the horrible math teacher I had who completely destroyed my confidence, but that's a whole other story). Anyway, I had a tutor, but I only saw her once a week, and in the interim, my parents would have to help me with my math homework. With my mom, it was fine, because she'd always trick me into understanding the work by getting me to explain to her the parts that I *did* understand (because she was even worse at math than I was), but my dad's always been good at math, so he'd go over and over it with me, and insist on making me re-do my homework over and over again until it was PERFECT (even assignments that I'd already completed), and when I didn't understand, or wanted to just be allowed to finish it on my own and go do something else (like play my clarinet, for example,) he'd tell me I was "stupid" and "lazy." Once, he got really angry and yelled "You don't have a F***ing clue what you're doing!!!!"; when I didn't understand a particularly difficult assignment. He'd even go through my backpack and my books to look over my math, and then he'd yell at me if I'd made mistakes or not finished something, even if it was something I was planning to ask Mr. Graham, my tutor, my mom, or an older student for help with later. Sometimes, my parents threatened to take away my music if I didn't finish my math, but they never actually followed through, because they didn't want to deprive me of something that I liked and was good at.....but it sure made me resent them, because it was my mother who had insisted that I take music in the first place, so I thought it was hypocritical for her to threaten to take it away once I started enjoying it.
Anyway, what am I saying? Oh yeah.......Flkwmn, it sounds like your son isn't really responding to having fun things taken away, so maybe that's not the way to go. Maybe the best thing to do would be to do the exact opposite, and show him that learning can be fun sometimes. For example, instead of saying "You can't go anywhere until you finish your homework," you could take him to various places that are educational, but also fun, in order to get him motivated. For example, if he has to write a report on an animal, you could take him to the zoo, or if he's learning about art history, you could take him to an art museum to look at paintings......better yet if they have classes for young people where he could actually try imitating the styles of his favourite artists. If you can't afford that, public libraries have programs for kids of all ages that they can participate in for cheap or free. Oh, and I don't know if you have something like this where you live, but we have the Ontario Science Centre in Toronto, which is about an hour or so from my parents' house. It's an interactive museum where kids can learn about various forms of science through experimentation. For example, to teach about static electricity, there's a giant electrostatic ball that you can put your hands on, and it makes your hair stand on end. Your son would probably find that much more engaging and fun than simply being told "Read chapter 3 in your science book and answer the questions about static electricity," and it'd probably be well worth whatever a day's admission to the museum costs now. Hell, buy a season's pass if you want, then you can take your son as many times as you want for a flat rate.
I'm not saying that these field trips should replace your son doing his homework, it's really all in how you structure it. For example, instead of saying "If you finish your report for Mrs. Blahblah, we'll go to the zoo," or "We're going to the zoo, but as SOON as we get home, you have to write that report for Mrs. Blahblah!!!"; or worse yet, not saying anything, but forcing him to do the homework upon return from the zoo without having made any prior agreement, you could say, "Hey, Son, I saw in your planner that your class is doing animal reports for science class. Any idea what animal you want to do? Boa constrictors? That sounds like fun, how about we go to the zoo this weekend and visit the reptile house? We'll take the digital camera, and then when we get home, we can print the pictures up on the computer, and you can write a photo essay about what you learned." I know this isn't always possible (I think math will always be torture for some kids, no matter what), but when it is possible, go for it.