Since he already works on harder stuff at home, would it be possible to give him some instruction at home (ie introducing new concepts - maybe using Khan Academy) and then he could work on his homework (that you give him or he does online) at school while the other kids are working on their math. Or maybe if the school wants to be more involved you could get with the 3rd grade teacher or gifted coordinator and work out some arrangement where he is instructed at home and then does work at school - using the curriculum agreed upon by the school with the agreement that they would help him if he had a question.
