He is popular with his friends and teachers, plays soccer and trombone, and enjoys reading and computer games. There are no problems here at home, and his teachers and coach know of none. I know he is bored with school and thinks most of the homework is "stupid," but I can't convince the school to give him more challenging assignments if he can't show the discipline to complete the current work. Is there anything you can suggest?
I also wonder if he is somehow feeling some peer pressure not to appear too smart. This problem can start to crop up at about this age level. Try an open-ended casual conversation about this topic and see what he says. How are his friends doing in their grades? Does he not want to appear different from them? If this is the case, perhaps he and Dad can talk about how guys can have different skills and still be friends, or what to say if he is being teased about being smart.
Is more challenging work from school truly a possibility for him? If so, have a teacher share with him what that might be to get him interested. He may think it will all be too hard or not interesting, so why try for it. I'd also like to point out that challenging material can also be in the form of special activities such as Junior Great Books, Science Fairs, school newspaper, and so on. What academic skill does your son have that might also turn into a fun activity with other kids for him?
All of these recommendations assume that there are no other problems for your son such as depression, a serious peer problem, or a health concern. If you can truly rule out all the serious stuff, then go for some positive reinforcement to see if he improves. Good Luck!