Can you request that he be able to do the advanced worksheets instead of the main worksheets? Ask if they can give him the end-of-the-year test. If he does well, ask if he can skip the math worksheets on the topics that he already knows, and just do the advanced one. Differentiation shouldn't equal more work.
In first grade this year, they are supposed to be giving my twins end-of-chapter quizzes BEFORE the chapter starts, and then substituting differentiated work for them if they already know what is being taught. Still waiting for that to happen; but it is supposed to start soon. (I have been told to be patient - they have to learn the class routine first.)
