You really need to call a lawyer, or go to legal aid, since, as a minor, he probably doesn't have much income. Most legal aid lawyers HATE the police. If he sues and gains any damages, he should relocate as quickly as possible. Someone in your family has to make the call whether the girl's father and the other cops are truly dangerous or just trying to teach a young man a lesson. If the girlfriend's account is true, she definitely has a case of child abuse.

Local cops are incredibly tribal and can't abide anyone questioning their authority. You must understand how fraught this situation is and that every movement and act on your part must be thought out beforehand. Personally, I think violence or preparing for violence with a gun is a bad idea. You know the cops will back each other up if there is any type of incident in terms of lying about what happened, or covering each other in a physical encounter.