It could be that he gets along better with older kids because they're "picking up the slack" and more tolerant of his social weaknesses, since he's younger. Kids his own age might be less tolerant.
Then again, it could be that he gets along better with older kids because he respects them more, since it's a better peer-match, and he gets frustrated with kids his own age for "acting like babies."
Without direct observations, it's hard to say. The feedback from the teachers is too limited to be helpful.
