My kids are both like that - unless they're challenged (or motivated in some way), they don't exert much effort. The director is probably right that your DS needs the 4-6 grade curriculum.
My daughter was PLAGUED by perfectionism when she was younger and the thing that finally worked was putting her in an environment where she couldn't achieve perfection and requiring that she just deal with it. It was tough, but we pushed through and now at 10 she's so much calmer and less stressed it's like she's a different kid.
I would talk to the school again and see if there's any way your DS could be moved to the 4-6 group.
I strongly agree with requesting that he be moved up now. A good Montessori should be flexible, he probably has reached the limit of the room he is in, and I think that being placed where the challenge is greater is probably what he needs. My dd(now 12) could certainly have benefitted from moving to the 4-6 room sooner.
