Sounds like son has anger issues.

Is there an positive activities you can help him use to get out that negative energy? I think sports are really good for kids. Various maritial arts programs help kids learn discipline and the proper use of force. Some kids just need more activities than other kids.

Is son exposed to violence in movies, music, video games, etc.? If so, try to replace those with more positive programs.

My nephew was very violent with his sister until he saw his mother get into a car accident and was in the hospital. He just didn't understand pain and suffering until he saw his mom hurt. He is a sweet boy now.

About calling the police, if you threaten something like that, be sure to do it. This would be the perfect age to scare him with reality about consequences for actions, commit a crime (i.e. assault) will get arrested.

Good luck.