I started him at fourth grade, because 4B (end of fourth) is as far as we went in Singapore Math before
My DD's public school 4th grade math covered SM through about 3B; my recollection is that the age / grade difference is such that SM Grade X is most likely equivalent to US public school Grade X+1. So I'd call your DS a 5th grader equivalent.
If you look at the ALEKS placement recommendations (
http://www.aleks.com/k12/course_products/recommendations), an advanced 5th grader should be in the ALEKS Lvl 6 course.
I'd guess your DS is placed possibly as much as 2 levels too low, which would explain both his rapid progress and his dislike of repetition. (FWIW, if I were placing my just-finished-4th-grader in ALEKS, I'd put her in Lvl 6. I've looked at the scope of that level, and she's familiar with the first couple of items in each area.)
