I know other parents who have gone the Montessori route and been happy but it probably depends on each particular school and how they have things structured. I wouldn't move or do anything drastic unless you are able to talk to them in detail, ask specific questions about how they would place him, and possibly try out the school (some schools will allow kids to attend for a day or two without enrolling).
We had the same issue with our DS and math and decided that even if they moved him up one grade for math, it would still be way too easy. I advocated for having him do work within the regular grade level classroom at his own pace (for instance doing advanced workbooks/khan academy), but he wasn't supervised very well and ultimately we decided to move him to a school that is sending him 3 grade levels ahead for math (luckily the school has their math schedules aligned).
