American education system has its own value. They are lots of good scientists, Mathematicians and engineers produced from the US education system. Don't be cheated by other people's saying. The US education system is based on personal passion. If your son like math and has math talent, he should be great in American education system too.
Your son is too young to make decision. He will regret when he saw so many homework in China.
Also math is not everything. Do you still use calculus in everyday life? Social study and logic reasoning are also very important.
