DS hasn't worked systematically through the other books and the only other course experience he has is the current AMC10 course (bit on the easy side so far but he's loving it btw). My uninvestigated guess is that it's mostly harder because it takes several ideas to solve the harder problems, where even in AoPS algebra problems are normally one idea plus follow your nose. But of course, it'll make a difference whether your nose is habituated to pointing the right way...
Concretely, I do remember an AoPS geometry question assuming children could easily factor a quadratic.
In your place, I'd let your DS do things in parallel as much as he wanted, but be prepared to intervene if it did become apparent that he needed more algebra at some point.
