Why?
I personally think he's gonna change football, especially at the NFL level. He's around 6'1" and wears a size 15 shoe. He's gonna get bigger (I'm guessing around 6'3") and he'll get stronger. He's fast as hell and can make plays with his arm while stationary and on the move. As mentioned above, he's only 20 so he'll get smarter as well, especially playing in the SEC which are more like defenses he'd see in the NFL.
I'd be astonished if he wasn't the #1 pick when he comes out. How he performs next year in the SEC after gaining all the accolades this year will go a long way to proving whether he'd be a viable NFL QB.