Here's an idea (one that we've used in order to keep DD more or less in synch with what the school thinks that pacing ought to be, anyway):
can you have him do math at his preferred rate, but only have him work on it for one or two days each week?
I used to worry about the effect of not doing a lot of practice problems, but the bottom line is that DD really doesn't need to. I mean, she knows HOW to-- and does do so fairly graciously (at 14yo) when she encounters something that she needs to dust off or has more difficulty mastering. So a couple of practice problems with the concept, an assessment, and she's got it.
I've never tried to control her innate learning pace, per se. I tend to think that is a losing battle. If I force her to work at a pace that is too far out of her comfort zone, then she goes into refusal and underachievement.
If you were to restrict math to a day or two a week, and then maybe introduce competitions math or something else to "fill" the other blocks, would that work?
I hear you about group instruction. This is something that my DD has found very disheartening about online math.
