Since the point of the exercise is to get your son into 6th grade math, I'd pose them this question: When 11-12yos enroll in this school for the first time from public school, do you send them back a year in math? Do you send them to a 5th grade class for it?
I would expect the answer to be "no," at which point I would demand equal treatment for my son, because he has been working at 5th grade math the entire year before, just like those kids, and setting him back would be a punishment, and a severe blow to his confidence, just like it would for the older kids. The fact that he's 9 has no bearing on the conversation.
And then I would say my DS is ready to take whatever mid-term fifth grade math test they're willing to give him. When they start the "gaps" nonsense, I would interject that this is a non-problem, as he has supportive and involved parents at home.
I would not bother challenging their beliefs about their curriculum, because the ability for humans to change their beliefs based on data is extraordinarily rare. That conversation can only annoy you, and is unlikely to help you achieve your real goal.
