1) You are allowed to send him to the new school if that is what you feel is the best choice for your family. You are the parent, and you are in the best position to evaluate risks worth taking.
2) You can hire a tutor for Math, perhaps the school will even allow the Tutor on campus during regularly scheduled Math time. Or sign up for afterschool time at a tutoring center.
3) Look into Art of Problem solving for Independent study when you son is ready for it.
4) You can strongly ask for your son to be sent to an older classroom during Math class - you have taken all the right steps, and been patient, so now it's time to state your case in writing and strongly ask for subject acceleration.
5) You can consider asking for a full grade skip. You can offer your son a grade skip at the current school vs. same grade at the private school vs. an hour of afterschooling each weekday.
