That man is not part of this flash mob. That unnamed 40-something sees the sudden dance party, and even though he doesn't know the choreography and clearly doesn't know what a flash mob is, he tries to join in.
And he never stops.
He's not there to ruin it, or make fun of anyone. He doesn't even seem to realize that this was choreographed at all. He was so caught up in the magic of a dance fever that spread from person to person that he thought he could catch it, too. He's so excited to be part of what he thinks is a legitimately spontaneous moment. There's a little bit of terror in his eyes, because he doesn't understand why everyone is so much quicker at picking up the new moves, but that doesn't stop him from spending the entire video trying to catch up. He doesn't even stop when it becomes clear that he's the only one in the dance who isn't a young college student:
This is a man who saw everyone burst into spontaneous dance, and he didn't think "Oh, a flash mob, or a pep rally, or a stunt"; he thought "Oh, life is a musical now? OK, I'd better adapt!"