If you think your DS would enjoy more of a challenge in math, I would definitely adovocate for that. Your school sounds similar in ours in that there are flexible groupings. So a good percentage of 5th graders are either doing 6th grade math in the 5th grade classroom, or they are actually moved to the 6th grade to do more advanced math and they ability-group there as well. I don't think there is as much movement in the lower grades but DS's computerized achievement test results were so high, they decided he needed an out of the norm placement, so last year in second grade he went to a 5th grade classroom and was doing math at a 6th grade level. Now he is going to a 6th grade classroom to do math at some advanced level (who knows what--I forgot to ask). It has worked out very well because they have the math schedules more or less aligned with each other. I would ask about above-level testing to see what material he can handle, if there are gaps, etc. I wouldn't let it go just because he's not complaining. He probably is settling for the lower level math and doesn't know any better.
