If he is at least 18 he is considered to be an adult, and you are no longer required to provide food or shelter. You mentioned rent. If you have a contract, he has obviously violated it by not paying. If there is no contract, no legal action is required. You can simply force him to move out or press charges. If he's the deadbeat you seem to say he is, there is no diplomatic solution to this. Squatters can be extremely manipulative, and will take advantage of any kindness you show.