I wonder if it would be possible to keep DS in the 5th grade math class and then have him do a little Aleks on top of that to fill in any gaps (his schedule is flexible enough to do both, but would it make sense curriculum-wise?). Thoughts on this idea? The following year he could either go to middle school algebra (gulp) or do AOPS pre-algebra.
We did ALEKS alongside instruction in the earlier grades (DS is now in 5th). The only thing that should have been better planned was how much to give him, and how to slot him back into the school's curriculum correctly. He ended up whipping through several years of ALEKS really fast; and then we had to face the reality that we were not going to send him to the high school as a 5th or 6th grader, which means that there's no place to put him back into a math class (and class is the right place for him, not independent study).
I would choose to go deeper or toward problem-solving rather than faster. At some point we diverted him off the "straight ahead" track into math application work when it became clear that it made no social/emotional sense to let him run ahead indefinitely.
DeeDee