The problem is more than just a computer, its happened before and now its happening again. If he flew through 8th grade he's clearly ok with the work, its just a matter of him buckling down and doing it. I think maybe you should help him aquire and maintain a work ethic before you go punishing. Let him have the computer, but sit down and spend some time with him, make sure he does homework and tries with school.
I was able to make it through high school with little more than just paying attention in class and scribbling down some homework in homeroom every morning. But it didn't start out that way, I had to learn how to get the work done, how to take the right notes, and how to hand in the important stuff so that my grades would be As or Bs.
Teach him to work! Reward him with the computer, don't punish him with it. Then again, he's what, 14, 15? He should probably know better by now, he is kinda a bitch from what I see.