OP, think about where you want your child to be senior year of HS and work backwards. The various math tracks in APS from 6 -12 are on page 55 and 56 of the program of studies, which can be found here.
http://www.apsva.us/cms/lib2/VA01000586/Centricity/Domain/73/FINAL%20Middle%20School%20POS%20CWS%2011_19_12.pdf
The track in the top level of page 56 is basically for the most advanced math students (like the MIT/Caltech bound types). Most strong students will be competitive in college admissions by completing the second hardest track, which results in taking BC Calculus in 12th grade.
Anything outside of these established tracks will be a one-off and I agree that you should speak directly with the school about this. I will note that the very, very talented 6th grade math students in my son's MS this year were taking Math 7 for 6th graders. This was a small number of kids, and if they stay on this track they will be taking Geometry in 8th grade and so on.
I'm not trying to imply that your son isn't exceptional, but is he really so exceptional that he should be on track to take Multivariable Calculus in 11th grade and then some upperclass college level math as a senior? This is the track for kids who are truly math prodigies. Is this your kid? Maybe, maybe not but why not just keep him on the normal "hard" track and let that be his easier class. He might need something to balance out the work in AP Government or Latin or whatever.