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. Also look at their Intro to Number Theory and Intro to Counting and Probability courses.

My son did the books and is now repeating the courses via their online option. He is in 6th grade and is finishing Algebra II.