To be perfectly fair, since your friend thinks he's a vampire, it is also possible that he was bitten by someone else who claimed to be a vampire. There are people who live a sort of vampire lifestyle and who drink small amounts of blood from others. Doesn't make them vampires, only makes them fantasy role-players.
Of course they are simply living out a fantasy. The whole idea of vampires began in the Middle Ages with mistaken ideas of what happens to the body when you die. And in the 1990's it became trendy and fashionable, a trend that has continued with the "Twilight" series and tons of fantasy novels.
But the fact is that blood-drinking vampires and being under the " ownership " of another vampire. So your friend is either deluding himself or he is trying to make others believe things that aren't true.