Sympathy! TBH, testing's just going to tell you he's too bright for the test to be reliable, isn't it? I mean, in the most informative case it tells you he's much stronger in one area than in another, but then so what?
Maybe it would help to take the bull by the horns and talk about what kind of disaster you feel he's one step away from?
You say you can't just keep piling on the work: well, what if you just stop whatever you're doing to pile on the work, for a month say? Call it a consolidation break and let him do what he chooses to do, intellectual or not, but don't provide him with any new school-type work for a bit? Or institute a "break between courses" tradition, so that e.g. when he finishes Pre-Algebra he doesn't get a new course for a month (but can play with mathematical puzzles, read maths books etc., if he chooses)? I think the latter is what I might do if I were homeschooling my DS. There's something to be said for periods in which one can choose to play with a subject without the training wheels of a predefined course; and if for some subjects some times he chose not to do anything with the subject in the break, no big deal.
