If you are considering toe-dipping, I'd recommend starting with pre-algebra. If he's already had pre-algebra *and* has mad and well-developed deep problem solving skills, I'd consider algebra but AoPS pre-algebra will not be the same as a standard pre-algebra course. Perhaps the pre-algebra 2, rather than the first one if you wish, but it would be kinder to start developing perseverance with harder problems with material he has some familarity with rather than also learning a new subject. And this is really, really different than typical math.
Now that you say this, I wonder why I didn't also suggest considering it :-) DS had in principle done the geometry material a long time before he did AoPS geometry, after all, and still got challenged. One thing to note is the "full refund before third class" policy and the fact that there are Prealgebra classes starting in February. So if you're not sure whether Prealgebra will be right or will be too much repetition and AMC8 would be better, you could sign him up for a prealgebra course and bail if it does turn out to be too easy, with time to enroll in the AMC8 course instead in that case. If the geometry course is typical, you'll see three sets of challenge problems in the prealgebra course before you reach the point of commitment, which should be enough to make a good judgement of whether it's suitable.
