Did the GT teacher who proctored the test for your ds have any advice re which math class to place him in?
I'm not familiar with MAP testing, but fwiw I would think with math it would be fairly straightforward to compare the curriculum he's already learned at home vs the curriculum taught at your middle school. I'd look at the course title more than what grade it's taught at - for instance, in our school district typical kids take a 7th grade math course in 7th, pre-algebra in 8th, algebra in 9th, geometry in 10th. Honors track kids take pre-algebra in 7th, algebra in 8th, etc. Gifted program kids who have subject-accelerated take the same curriculum but earlier. If you look at the curriculum for pre-algebra your ds has mastered this year in homeschool and compare it to the school district's pre-algebra curriculum, are they comparable or are their holes? If there are holes, can you focus on making up those areas during the rest of this school year? If there aren't large gaps and if you feel he's confident in what he's learned in pre-algebra, I'd advocate for putting him directly into algebra. I wouldn't put him in geometry (which I think one poster above suggested?) - he needs algebra first.
Back to the MAP scores, I find the percentiles a little tricky when it comes to math curriculum (but again, I'm not familiar with MAP testing so I may be just clueless!). The thing I think is tricky is they are fitting a score into a percentile of students in grade 7 or grade 8 or grade whatever... but typically once kids get into middle school not everyone takes the same level of math course in each grade... so I'm not sure how you can relate the percentiles to actual course placement.
polarbear
ps - I just remembered that there is a test called an "Algebra Readiness Test" that is offered in our school district, as well as (I think) many other school districts in the US - I'm sorry I can't remember the name of it at the moment! It is used in our district to screen which kids go into algebra in 8th vs waiting until 9th, and also used to screen kids going into pre-algebra in 6th which here is considered to be 2 years ahead of grade level. I'll try to find out the name of it - it might not be something available where you're at, but if it is, it might be useful since you are specifically looking at pre-algebra vs algebra.
