If this guy isn't someone you know terribly well, you might have to do a little detective work. The obvious place to start is to see what kinds of school activities he's involved with. Sports, drama club, the school paper, whatever it is -- they're all little clues to the kinds of stuff he's interested in.
If the two of you share some of the same friends, that's another potential resource. Ask them what kinds of things he's into outside of school. You never know -- you might both have a burning passion for mountain biking or speaking German. And even if the two of you are into completely different things, you'll know a little bit more about what makes him tick. Just don't go overboard with your reconnaissance; there's no need to interrogate everyone he knows. Talk to people who know both of you well and won't broadcast your interest to the entire school.
Your investigative work will come in handy on the phone for a couple of reasons. First, you'll put him at ease by getting him to talk about stuff he loves. Second, your interest in his activities is a good way to let him know that you want to get to know him better.