C.J.
I might be telling you what you already know, but it is good for him to be the most advanced/oldest as well as the youngest in classrooms. It allows the kids to explore different roles, etc.
My youngest was in a similar situation where all his classmates moved up and it was him with 2 yo and he had just turned three. He is the youngst in our home, but now he had this new role of being the "big boy" he did do a little whining, but the teacher picked up on it immediately and began reinforcing how he was the oldest and the helper, etc.
Really made a positive difference. Maybe a convo with teacher as a parent, not a coworker, of behavior you have noticed. has she noticed it too? What are some solutions or ways to address it in the classroom and at home for consistency?
I bet you all will come to a great solution and it might even be - he moves up a class!