Sorry to hear of your predicament. I would not call the police as this is a civil matter and not a criminal matter. I would take pictures of the damage to the car, take pictures of the freshly chainsawed tree. Get the contact information of your neighbor that doesn't want to get involved so you can have him served with a subpoena to testify to what he knows. Perhaps he'll be more forth coming while under oath in a civil court proceedings, where he is required to tell the whole truth and nothing but the truth so help him... Have your son file a civil complaint in small claims court and take it from there. That is what I would suggest.