We had just watched the movie a couple of weeks before, and he was super excited at his first theater/live performance experience. Yes, that wide eyed expression is his "I'm Bursting With Excitement" face.
Plus, he knew the songs, and could say "Supercalifragilisticexpialidocious" flawlessly :)
After about 10 minutes, we moved to some empty seats up at the front ( 2nd row! ) where we had a spectacular view of the stage and performers, and the little boy was very intrigued by the musicians/orchestra hidden down below the stage, where the audience couldn't see.
All in all, he was very good for the 3+ hours that the show went on for - even I was getting a little restless - and a good time was had by all.