In situations like this, the only thing that can help you now is confidence. It's hard, I know. This coming from the most shy person ever, but if he's showing signs of interest then that's a good sign and if he's not interested then he's not worth thinking about.
I'd suggest casually trying to get into a conversation with him. Considering you're in some of his classes, do have any mutual friends that you could possible stand in the same circle as him with? Even when doing something like handing out sheets or walking around the school, smile at him whenever you have the opportunity. It doesn't have to be big, just even a shy small smile. If you can have any contact whatsoever that's great! Then you can maybe build up some courage to message him on Facebook (if you have that) and it will continue from there, good luck!