Contact the authorities but build a strong case against him first. The last thing you'd want is to report him to the police, them find nothing and vindicate him. Do the following and let me know how it goes
- Record all contacts between him and your family and pay attention to threats. Threatening you with violence or blackmail is a crime and if you have evidence, he will be convicted
- Document facts about your initial interaction, including your insurance claim details, things you can remember, and photographs you've taken of your own car's damage. If you can establish he was in the wrong here, it will go a long way to support your complaint about the threats
- Once you have plenty of evidence of clear threats or blackmail, go into your local police station. Make a complaint and provide this supporting documentation. The cops will investigate this, review the evidence, independently corroborate it and charge him. He will be convicted and so will anyone else threatening you.
I have a question though: Didn't you report the collision and didn't your insurance agent follow up and establish fault? I don't understand why you just changed cars and moved on. Whats going on here?