Oh Val - that's horrible ! Not only does it interrupt your life you don't know if he's watching you or not.
I agree with everyone - start being tough - maybe you can simulate some clicking noises like your phone is being traced ? What a jerk that loser is but I'm still scared for you - I would def change your number - I have an unlisted number and it's not so bad and we never had problems with obscene callers anymore -
GOOD LUCK !