I would think that the advanced work will be better for you child, and would outweigh any issues his slightly immature behavior might bring up. Let me ask you this, Does he get embarrassed easily? My older son is immature for his age, and it comes out in school. It doesn't seem to phase him what other kids think of him. They don't get him, so what does their opinion of him matter?
But, with that said, I find kids very quickly adapt to their new environment and expected/tolerated behaviors of those around them.
