especially he is so smart with math. Most likely, his social skills may not be as advanced as his math skills. But as you might agree, social skills are more important in life at every stage. In elementary school, good social skills (maturity) might mean that your son won't be the target of bullying (a big topic in past few days here).
Plus, he has a long life before him, what is the hurry of rushing to school a year earlier? Who would care if he is 22 or 21 when he graduates from college? Right?
I never understand why so many of my Chinese friends here rush their kids to school, and keep pushing the school to let their kids jump grades, at the expense of much needed natural developmental growth of children. And very importantly, we as parents need to provide the chance for our children to enjoy "childhood", playing in the mud, doing something silly or funny or even stupid......
If I may, I suggest your son join a hockey team. Hockey is the best sports for "mental strength building" in my opinion. It has worked wonders for my son. Both his physical and mental strength have increased dramatically.