Pretty sure it's a pair of glasses, and what we're seeing sticking out of his fist is the earpiece. You can see him drop whatever it is as he's falling, and it lands about 5-10 feet away roughly in the direction of the statue. About a minute later in the video, while they're working on him, you can see a pair of glasses lying in that area.

That said, as fast as things happened, the shooter could have seen a flash of black in the guy's hand and thought the same thing we did, that he had a knife, and that's what drove him to defend himself. That and the group of people trying to brain him with skateboards, of course.