Stick with the plan that your son and the administration likes; sounds like the G&T teacher is engaged with him.
If they aren't engaged in math competitions, then I'd think that would be the only additional piece to look at as increased complexity of problem solving seems like a greater thing to progress in than linearly pushing forward.
Percent score on an acheivement assessment is a standing relative to students at that level rather than a percent of material mastered.
My DS7 is at a similar range, and he would really like to be getting more math at his interest level from a teacher, but he also finds he really enjoys video content in math.
