Nautigal, this has always been the sticking point for us with subject acceleration. Something always scotches the schedule. Our current plan is astonishingly complex... it will be miraculous if it works... but it could. So we're doing it, because DS will finally have appropriate math in an actual class. But it took moving heaven and earth.
I hope your DS can try the class with the enthusiastic GT teacher; if it's good enough, maybe it's good enough to have recess for? Where will he be when his regular class gets math-- can he go to recess with another class then, and would he consider doing so?
Good luck,
DeeDee