When someone says he is the king when he is not, he is probably a mythomaniac. He is insane and because he is delusioned, he lied and believed that he is telling the truth. The best way to cure or treat a mythomaniac is not to argue with him but to question him with facts. Like asking him 'Where is the Queen?' or 'Where are your Ministers?'. He will not be able to answer those fact and will probably submit that he is not what he claims to be. Have you seen any familiar famous people behaving like a mythomaniac around town recently?.