[niels_olson]

Depends. At the best, you're being paranoid. At the worst, he could be developing schizophrenia. Is he in his late teens or early twenties, does he sound agitated or paranoid? Is his thought process coherent? What is he talking about? A young, paranoid male talking gibberish to himself without regard for who's around would be concerning. If you want a great portrait of schizophrenia, watch "The Soloist": http://www.imdb.com/title/tt0821642/

But if he's not acting like that, then maybe he's just helping you expand your definition of normal.