@emwalden,
I think you will find that if you refuse to allow the school to hold your son back, they will capitulate, but you should consider whether or not it will be good for him in the long run.
If, for whatever reason, your son has not absorbed the lessons taught to him in this grade he will have difficulty in the next - especially in Math.
There is a stigma attached to being "left back," but not so severe as it was in the past and 10 years from now, no one will car one whit.
What is best for his self-esteem: Struggling to keep up with the other kids and constantly failing, or being able to compete with his peers and succeed albeit knowing he was "left back?"
Think about what is best for him, not for you.