I suppose it can be siphoned down to whether or not your kid can understand the notion of delayed gratification, ie that schooling now equals a more comfortable life down the road. If the kid can appreciate that, awsome, he'll do fine. If he can't, reward him with *something*, anything. Money? Maybe. Trips to ball games or movies or game arenas, something to have him associate good grades with personal benefit.
As for being upset that he's not learning just for the sake of learning, well shit, in most public schools he's not learning a lot, period, and there's no point in "learning for learning's sake" when they have no control over their curriculum (not that they should, at such a young age, but they don't have any influence about it at all).
If you want your kids education done right (or, I personally imagine how I'd like to try when I have my own), homeschool him. Worried about his social skills? Enroll him into public school for *just* two classes, one before and one after lunchtime.
Personally I'd go with whatever two foreign languages the school teaches, because I couldn't homeschool him in that myself.