I have given some consideration to it being faked, and I have decided that I really don't think it is.
If you look at this, he doesn't flinch.
1812 Richard Spencer Overture - YouTube
See? He doesn't flinch at all, even when the guy does put his hand on his shoulder first.
Richard Spencer punched for every syllable in Nazi Punks Fuck Off - YouTube
He'd be more nervous. He'd be looking off to the side, and he'd definitely flinch if he knew it was coming.
I'm willing to concede that the guy just isn't very good at punching, though. I probably wouldn't have done much better.