If your son has gone through the AoPS Pre-Algebra book, anything he does at the school will not be a challenge for him. The online courses are more challenging and different from the books so you might want to have him do that.
Side note to ruazkaz - which AoPS course(s) did you find so different from the text? We've only done Algebra I, but it was 100% the text, plus an extra ~10 challenge questions each week. I had been assuming this would be the same for the other courses for which there was a text?
To the OP, I would agree that if your DS has done all of the AoPS pre-Algebra text, they've probably already gone further and deeper than his school will, and he's ready for Algebra I academically, if not logistically. Perhaps there's some way he could do AoPS Algebra I while in his regular school, rather than travelling to another?
Some of the different views above, however, cause me to ask the question, has he
fully done AoPS pre-Algebra, or just the basics? If he hasn't yet worked through the full set of challenge problems provided, then he could probably gain a lot by going back through the book in detail - perhaps while sitting in his school's pre-Algebra class, but while he focuses on the AoPS pre-Algebra challenge problems related to the topic at hand.
We are in fact (re-)doing something like this: DS 10 and I whomped through the AoPS pre-Algebra text in about 6 weeks last fall to get all the basic concepts, which were mostly new (he had only grade 4 regular math at the time), but which he grasped easily. We did all the associated Alcumus pre-Algebra questions but very few of the problems in the book. We figured we had "done" AoPS pre-Algebra. Nyet.
We then did (most of!) Algebra I on-line as an after school (though we couldn't quite keep up to the end). This is where I learned that the Alcumus problems are pretty straightforward applications of the concepts, when compared to what's in the text. We are now going back and re-doing the challenge questions in the
Pre-Algebra text. Although DS grasped the concepts easily enough the first time, and didn't have conceptual difficulty with the Algebra course either, I am finding it worthwhile to go backwards and REALLY learn how to apply these seemingly simple concepts in all sorts of devious ways. I am humbly recognizing the need for depth over speed here, before we go on to Algebra II.
