So far lex does not know. However, in one of the episodes Lex got split into two Lex's (one good, one bad) The bad lex knew about his secret. Then, once they got together, lex forgot.

Lex has his suspicions, but nothing concrete to base them on yet. He's turning badder and badder by the episode, Lionel's influence and his belief that everyone is hiding things from him is really starting to erode his good self.

Lex is already having suspicions during the very early seasons so I would guess he finds out for sure around season five or six. I don't think he's 'evil' yet but he is getting to be, and Lionel's influence isn't helping.

There's 4 seasons on DVD, five is being aired now and I think they will be renewing smallville for a sixth season.

Hope that helps