Pretend he's new to your school.
What would you say to him then? How would you approach him?
If you pretend that he's new (and you've already had the basic where're you from kind of chat), then what else would you talk about?
You might ask him what he thinks of some of the teachers, or of upcoming school events. You might ask what his favorite non-school or sports activity is. You might try to find out if you have any friends in common.
Smile, be friendly and be yourself. Be ready to talk but also be ready to listen.
You'll do fine.