The cop may have been yelling, as he is supposed to, but the police officer was in the right. The boy was told multiple times not to put his hands behind his back, as he was approaching the officer he did exactly that after being told straight up that he wasn't supposed to. The police told him that if he puts his hands around his back it will be considered a threat and they will take action.
It was the boys fault for not following those instructions exactly. If you're crawling, there is no need to reach behind your back.
Police Officers risk their life to do what they do, as far as they knew, he was very well armed with a knife or a gun. If somebody doesn't like the police, then don't call them when you need help. One less richardhead the force has to worry about.