He is adorable and its wonderful. He clearly plays to his audience and I love his facial expressions.

My guess is that when he tries to talk to his peers, like he does on this video, he gets a blank look back from them as if he is speaking Greek to them, which he is. So he does not play to that audience anymore.

In Fencing class, its something that he does that is on the same level of the others and hence he can relate with them.

His size may just be related to his intensity levels and his posture to the same reason.

What a happy, loving son!