I would love to hear other opinions on this, but my understanding on cluster groups and differentiation is that they aid kids who are working about a half a grade level above the regular kids in the class. In my DS9's school, the teacher will typically group the kids into 3 groups: kids who are a half a grade level behind, average kids, and kids who are a half grade level above the norm. At the beginning of the year, say 3rd grade, you may therefore have kids who are performing at midway through 2nd grade, kids on track at the beginning of 3rd grade, and the more advanced kids who are performing halfway through 3rd grade.
I'm sure that this is dependent on the individual school and the individual teacher. We have been told by DS's teacher that she could not "step on the toes" on the next teacher by providing DS with any material outside of his grade level.
I hope other kids have had greater success with differentiation than we have had.
I would suggest that they test your child to see where he is in the math curriculum. Our discussion with the school began when DS's 2nd grade teacher gave him the end of the year tests, which he passed easily. It is hard for the school to argue that the child should sit in a classroom if the teacher has proof that he has already mastered the material. There is also not a great deal of waiting around and stalling for tests to be ordered: Your school's teachers already have these tests ready and available. You may therefore suggest to the school that they give your son the end of the year 3rd grade math tests or several of the math test during the year to gauge his math knowledge. If he demonstrates knowledge on those tests, then you have ammunition to ask for additional testing (like the Woodcock-Johnson III achievement tests) and subject acceleration.
Just remember that your child's school may not have seen a child before who needed additional services beyond their normal differentiated classroom environment. You may need to educate the educators before you can proceed.
