Is there an acceleration option in this school? It sounds like he changed schools (MAP was administered at a different school).
In our district, the current 5th grade math acceleration is based upon a matrix, which includes the MAP. You need 11 points in this matrix to skip 5th grade math and go to 6th grade math (which is taught within the K-5 school). A 94th percentile MAP will get you 5 points, 96th+ gets you 6 points. 90%+ on the 4th grade end of year test gets you 5 points (there are a few other ways to get points). If your school has a similar system, I would suggest taking the 4th grade end of year test.
I can see the value in learning multiple ways to get to an answer. Not all the methods have value - such as the ones that take 20 steps to do a problem that could be completed in 3 steps - but knowing and understanding multiple ways to get to an answer can be helpful as he gets to critical thinking problems.
If there is no option to accelerate, then I would probably just supplement at home. However, if you think he is ready for pre-algebra/algebra, there should be tests he can take to place into those (assuming there is a procedure in place to bus him to MS to get to the class).
