How should I let him know that I'm interested?

In situations like this, you should be cool and calm, and talk to him. If he stares at you all the time, you can tell he's interested, so you might as well talk to him. Take action, because nothing's going to happen if you don't take action. You should really just get up the courage to go talk to him. If and when you do, you should act really calm, and just be an overall nice person. Make friends with him. Don't get too nervous.