This is just cruel. The school should definitely offer him the opportunity to take at least Algebra-1, before claiming he can’t handle it. You could have him take a test to prove to the school he’s ready. I know RSM has course tests he could take.
Stick with RSM, and maybe encourage your son to enter math competitions/tests to prove he knows the subject material. If the school’s still obstreperous, change the school.
And I agree with @momofboiler1 : maturity has nothing to do with mathematical aptitude. In some cases they’re even opposite. 