Welcome to the Forum,
I'm very sorry for what you are going through, it's especially difficult because you have a child with this abuser. If you are in the states I would also contact the State Attorney General and advise them of what is going on. It never hurts to get as many people involved as possible, let them know everything that you've been told by the police, i.e. that threats made over the phone are not really threats. I know that they are! That's ridiculous and they should be right on top of those calls! This man has a dangerous history.
I agree, you do need someone to help you go through this process because you are being worn down and out, that's what abusers do, that is the purpose of the constant harassment. I can see here just from the little that you have written that he is abusive not only to you but to your son and I believe at a certain age your son has every right to say he no longer wants any contact with the abuser. I would certainly allow him to make a report if wants to and look into his being able to block all contact.
It's good you are here. Please take good care of yourself.
Peace,
Rain