It doesn't seem like an unreasonable pace to me. DS6 did two years of elementary school math in less than 4 months before we hit the times tables wall. So Algebra I in that same space of time--especially if he came with some knowledge beforehand--seems reasonable.
If you're doubting that he really had 1/3 of the concepts under his belt already, well, that, too, seems possible to me. I'd bet money that DS6 has some of them already. Your DS is so far ahead of mine, it makes perfect sense to me that he'd have 1/3 or so.
But I'm just extrapolating...I know nothing for sure, of course.
