They did give the end of year assessment as a pretest and DS scored 75% with mistakes more related to rushing or lack of exposure - e.g. not knowing the sum of angles in a triangle. So I do think he needs some of the sixth grade curriculum but could easily move through the material in a month and be ready to move on.
It was the 6th grade EOG exam they gave him? If so, I think you are right to be concerned. I have similar concerns for my fourth grade DS9. He's supposed to be in a gifted program, but 1/3+ of the fourth grade is in his math class and they are only doing the 4th grade curriculum. I am hoping to get some test results tomorrow and then plan to request a 5th grade placement.
You already have the test scores - I'd start thinking about the suggestions Grinity has given you. If your son is already getting 75% on the material he's going to learn this year I think you have a very compelling argument for subject acceleration. I don't know exaclty how it works in MA, but in my state you really have to dig to figure out how they score and scale the EOG exams. Passing is around 50% and there is no calculation required. They have one section where they can use calculators and another section where the questions require no calculation.
If you do a little digging you may find that 75% is even better than you originaly thought. Maybe you could ask the school what % of last year's 6th graders scored above 75%?
http://www.doe.mass.edu/mcas/2010/release/default.html
