Our son was in a similar situation and we pushed the school to give him more challenge and they did not. We kept giving him things to do at home and at the same time continued to push the school. Finally, his fourth grade teacher helped us push and he began to do independent study during math.
He is now in 10th grade and attends a local college for Linear Algebra and we are hopeful he will get accepted to a very rigorous school for 11/12 grade.
Every kid is different but for us the best things were the local math circle, math summer camps and math competitions. Good luck!
