funny as that is, I'd suggest following the advice to ignore the little tick. Until such time as he manages to borrow $100 from his mommy to get a cheap lawyer to send you something real. In my experience, ignoring people like this has a 95% plus chance of meaning you'll never hear from them again. Responding in any way will alert them to the fact that you can be wound up, and then he will try to get it moving in some way, if for nothing more than the fact that he obviously has a lot of time on his hands.
I would just ignore and not even respond. It is one thing to threaten and a much larger step to have to make official contact with him. In the meantime I would likely do some quick searches to find if they have any egg on their face elsewhere online. I would likely be bouncing/ignoring any personal email he sends unless it is from a solicitor and even then that could be faked.
Sometimes it's funny to see how people act and never realize that it's their own cause that get the result.