The officer's excuse is that he only had the training he had to rely on. Putting his knee into the 7 year old's back and trying to scare the shit out of him saying that he's going to get familiar with the juvenile system, etc. etc. Being an overall piece of absolute shit and he's just essentially cowardly blaming his training.
If you need training on how not to be human garbage to a 7 year old then you surely shouldn't be an officer. That officer needs some serious psychological help.