Still worth a shot, I'm sure the authorities can work back through proxies if they wanted too, otherwise next time he does it, save the post to a hidden forum, a mod room or the such where you can keep it as evidence, then contact the authorities.
They can work back through all the ip's and hopefull nail this sick scum.
I'm sure i speak on behalf of everyone when the end result is one less sicko chasing our kids.
Take care