Get in touch with your Mobile Network providor, their security team will trace the withheld numbers, you can then contact the police who will get in touch with your phone providor.
The police will contact him and fire a warning shot across his bow.

Hopefully this will be enough to show him you are serious about not taking any shit from him and its got a 3rd party involved..the police

Any further stuff and you can travel the route of a restraining order etc.