1st off, get proof BEFORE you call him that this stuff is yours and was up before he had his up. Anything that will hold up in court should work - the idea here is to make sure the guy doesn’t rip right back and tell you to stop using his stuff! But without proof that it is yours it is difficult to stop his claim, especially if he has tried to use things to prove that it is his.
2nd, once you have that, either you (or if you have any lawyer friends - much scarier when the letterhead is a bunch of lawyers) write a Cease & Desist letter. Sample:
http://www.free-legal-document.com/copyright-cease-and-desist.html Basically you are telling him to take it down, and NOW or you may pursue further legal action.
Just found this: you can try and get him kicked off google:
http://www.google.com/support/bin/static.py?page=ts.cs&ts=1114905
Also, I’m reading through this right now:
http://astuteo.com/blog/article/stolen_website_design/
Hope all this helps. I have to get to bed!
OH! Almost forgot

. I am not a lawyer, and what I wrote above should not be taken as legal advice. That is my general understanding of your best options. This is a disclaimer of ALL liability and everything that that implies. If you want legal advice, consult an attorney.