It does sound very stalker like and I have to say, if I knew someone had found out my details in that way I'd be horrified and want to stay well away from them.
Generally, I'd just be tempted to message him or try and get to speak to him if you go and see your brother or similar, I'm sure you can think of something (sensible) you could maybe ask him to get to talk to him.
But, if I remember right, you're still at school or were quite recently? So the age gap is a lot more highlighted (imo anyway), coupled with you being his mate, and flatmate's, little sister.
Maybe indulge in a bit of a crush in your imagination but stay well away from doing anything remotely stalkerlike and maybe look at boys closer to your age/more obtainable.