There are always going to be immature people even at university, sometimes they're like this to try and impress others- in other cases they are just bad people. If you really are close friends with that person then I am sure he won't turn on you, you might be overplaying scenarios in your head thinking everyone may turn on you. Just see how things are for a few weeks then if needs be try and move to a different flat as bad living situations can take an effect on studies. Take it you're in first year OP?