Was it a teacher or ao or the head of the department? There are several kids at our school largely because Middlesex refused to accommodate their math level. As I said, if your son is admitted, make sure you have a frank conversation with the head of the math department.
I personally loved Middlesex but from what I’ve heard the math department head is rigid and not accommodating. Math teachers are lovely and accommodating but they don’t have the power to approve math placement.