He was laughing at all of Trump's jokes and then suddenly realized the camera was on him. Probably hasn't been invited to a cocktail party since.
Holy crap, that’s up there with Clinton at Ron Brown’s Funeral.
I think he was responding to the sudden change in tone from Trump. Trump started out with some good natured ribbing aimed at Hillary, then started inserting some serious accusations at her. The whole room was laughing then suddenly gasped when he started delving into “off limit” subject matter.
I perceived it differentlythat omg expression seemed in response to something exceptionally aggressive that Trump said to Hillary.
Just watched it again, courtesy of another freepers link upthread. That guys look of astonishment came when Trump said, Hillary is so corrupt... (the guys eyes bugged out, and Maria Bartiromo looked around for the nearest exit in case a riot broke out), followed by that she got kicked off the Watergate Commission; how corrupt do you have to be to get kicked off the Watergate Commission?