Sorry. It's a badly gashed head. Not brain damage from being thrown to the floor and repeatedly kicked around the head (I've seen it TWICE at football) nor internal damage from stabbing.
I repeat. If the cop's out of order then he should face discipline. If the youth was being a dick then he should learn his lesson for next time. His hair will grow over the gash and a couple of paracetamol will sort the headache out. No-one died.