There's this kid in my homeroom that doesn't talk--?

He use to talk, but that was like four years ago, and he only talked to this kid who moved away. Hes pretty much managed to make himself invisible to everyone, but I notice him all the time. I've tried saying hi to him but he stares past me like I'm not there and he doesn't display any emotion. He sits by himself at lunch, I'm planning to go sit with him tomorrow. I know he's not gonna talk, but I would really like for him to grow to trust me. What should I talk about and what do you thinks wrong with him..?