If there is no benefit to your child, I would have him opt out of the 6th grade assessment if you can. You could explain how when he gets bored he tends to do less well.
As to taking the 7/8 exam now, I don't see why not. What do you have to lose? If he doesn't, he just takes 7/8th grade math next year. What is 7/8th grade math? Is it a pre-algebra class? There really isn't a lot more to learn in 7/8th grade math. Most math curriculum spirals, so there will be a lot more of what was in 6th grade math. Can you get your hands on the book they use for this 7/8th grade class?
My son is in what is officially the most rigorous math for my district, Algebra in 7th, Geometry 8th, Algebra II in 9th. All students take an exam at the end of 6th to see if they place into pre-algebra or take 7th grade math. Those that pass the first test at a certain level are given a 2nd test to see if they are ready for Algebra.
