Hi Target,
As far as this persons finding out the new phone number. It isn't difficult to do.
Where I live, all one has to do is pick up the reciever and dial 311 and you will get a recording stating what the number is. This can also be done from the box outside the house. There is a jack out there also.
If your son changes his number again, I'd find a way to secure the box outside.
As for calling the police. Do it, and keep doing so until someone listens. This person is a sicko and if he can kill a dog, whose to say he would stop at that?
I'm not trying to scare you. I guess I've read and watched to may true stories but it's always better to be safe than sorry.
Shari