How to get over someone you have no chance with?

Alright.. so there's this guy I have a crush on, and I know now that there is little to no chance of us ever making a go of that.

I only know him because he's the son of one of my parent's bosses, and I only see him when we go over to their house for dinners or to hang out with them, at which point he isn't usually down with us. He has a girlfriend, is two and a half years older than me, and we've spoken a grand total of /once/, for about three lines.

I could almost say that the only reason I like him this much is because I don't go to public schools and he's the only guy I've met even remotely close to my age since I've started being interested. It's likely just a young teen infatuation.

Nonetheless, he's on my mind often and, since I'm nearly positive that we won't be together, is there any advice someone could give me on letting it go?

No sarcastic remarks, please. All help is appreciated.