Trying to remember the name of a book

I read a book 5 or 6 years ago and can remember bits and pieces, but now I can't remember the name of it. It's about a teenage boy who moves into a new house (I think it was his grandma's) with his mother, and I'm pretty sure it was in Germany. He gets involved with this gang and cuts his hair short after he meets the gangs charismatic leader, whose name was ben-something. He gets involved in riots and makes molotov cocktails in his garden shed for some kind of 'revolution', but I'm pretty sure he is just being played by the leader of the gang. There is a girl that lives next door to him who I think he had a crush on, but in the end she has a car crash and she is trapped in the car. The car sets fire and he cuts her leg off with a chainsaw to free her. Any thoughts?