How he got the hold of your source code in the first place? Hacked your server? This is definitely unlawful.
Copyright is just a concept that what you make is yours (there is no need to register anything, or anything like that, the very act of creating makes it copyrighted). It applies to implementation only, not to ideas (ideas can not be copyrighted - exception is probably USA where even your dog can sue you, but for the rest of the world you can borrow ideas as much as you want
).
The best thing would be probably to threaten him to take it down and if not succeed then contact his server provider and next his ISP.
