If the police decide to charge and prosecute him they will, my brother went into the back of someone who suddenly braked behind another sudden braker and no police attended the accident but a week later he got a letter saying they are prosecuting him for "driving without due care and attention" and to send his licence off witha plead of guilty or not, he things that just the simple fact he must have been travelling to close to the car in front to stop in time proves he was driving without due care and attention so he pleaded guilty and is awaiting sentacing.
No idea what makes the police decide wether to prosecute or not, there must be loads of rear enders that don't get prosecuted. i hope its not because he is young (23).