WorkingMommy- it seems like you at least have the advantage of knowing what motivates DS.
I certainly think it is worth approaching the teacher/school to see if you can work out anything together (including, but certainly not limited to, subject acceleration, a different cluster group with higher performing students, and/or differentiation). These things may not interfere with his current social achievements and I'm sure if you get along well with the teacher she'll certainly be willing to consider your concerns.
If the school cannot or will not make changes, perhaps consider training for local spelling bees or even home competitions with family to foster that competitive spirit! Book clubs with older students do a lot to motivate my niece. She loves interacting with "intellectual peers" who appreciate the same books she does.
Good luck!
