Hi LG, welcome
It sounds like your are willing to let him try out 6th grade math and get his feet wet first. What's his opinion? can you get copies of the books for him to look through? did the school evaluate him? if so, what did they think?
My gut feeling is to take it slow for now, and see what happenes. You son is excited for this change, so that buys you some time to evaluate. Since he's already one year accelerated, and Math seems to repeat over and over anyway - at least this may be a different approach. I tend to think that a good private school "should" be about a year ahead of public school anyway.
Good luck either way -
Trinity
