We had a similar issue the first couple of times we had to call the police on Onyxx - they wouldn't do anything, but they did talk to her. She finally hit the "magic age" of 12. Nevermind that she was violent before that.
However - they did start responding when we started asking for transport.
At 16, with an assault/DV and drug theft record - they still won't do much unless we ask... And even then, you have to be insistent. For the safety of the other kids.
You said he is very careful of his behaviors when Dad is around... How does Dad treat YOU? Not necessarily the problem - but a possibility that he is seeing Dad do it and trying to "be a man". Especially if he is moreso than Dad is. Showing you who's really boss. As I said, this might not be it. But it's something that stood out to me.
Eventually, the police will do SOMETHING.