My son is only 6, but we had this conversation daily before pulling out and homeschooling. Now that I'm homeschooling, it's still a struggle to determine the right amount of work with the right level of challenge and keep the work interesting for him. He also complains of work being too easy, but then when we step it up it's too hard or too much. For my ds, it seems, it's a matter of learning to work at a challenge and to enjoy learning again. It sounds like your son is better at working hard; to me your son's situation sounds more like a combination of needing a little more challenge and the freedom to pursue areas of personal interest. I've also seen, through our own experience, how quickly a child can become an underachiever. It's hard to reverse, so obviously the earlier he can be challenged and become interested in what he's studying, the better! Personally I don't think MORE work solves the problem. It has to be more of the RIGHT kind of work. That said, I still haven't found the "sweet spot" for my little guy.

What does your ds love to do or read about? Maybe he could work on an individual project related to an area of interest. Would that be acceptable in his classroom? I know it's frustrating. Wish I could be more help, but I wanted you to know you are not alone.