In an exchange between Lionel and Clark at the prison, Lionel threatens Lex's life and Clark replies "he's your only son!" in protestation, and Lionel even agrees that Lex is, indeed his only son. No, he isn't! As established in season two, he has a half-brother named Lucas, who even has appears physically in the episode 'Prodigal'. However, he has not been seen since then, and the writers appear to be outright denying his existence now.