I am a pretty good reader of body language, you can't see it in this picture but his middle finger is straight up in the air pointing at you and your camera. The look on his face is showing that, he is just deciding if he should go ahead and make it part of the shot.
He won't ever treasure this picture, but you will always have it to remind him of the times when he was a darling little angel. Because we all know that the little boy angels have a streak of devil in them, myself included.