Yeah it's not photoshop. You'd need some pretty robust and expensive blending software to accomplish that. Occam's razor would suggest it's just a very flexible person, especially when considering this larger picture:
Adobe Premiere is affordable software to manipulate videos.
Occam's razor? The simple assumption is, "this is a faked video." It's not the unbelievable assumption that this person's hand is unique in human physiology- every finger, and finger joint is double jointed, and, he has the muscles to actually pull his thumb and fingers backwards.
One double joint, maybe, but 14 or more, one one hand? NO WAY.
This person would probably be on the news, if he could grip backwards. He'd be of great interest to scientists around the world. I say, don't believe everything you see on the internet. If it looks unbelievable, it's probably fake.