I'm very sorry to hear about this and agree that it's a good thing your son was around to answer the phone and perhaps scare him off.
I tend to agree with Queenb here.
While I do think that the guy is crying out to you to help him / talk to him (and is not necessarily capable of harming you in any way), I do think that its best not to respond to him at all.
I once had a similar thing with an ex boyfriend. I never responded to his call and never heard from him again, thank heaven.
Good Luck and keep us all posted.