One time a woman gave a note attached to a tip with her number... I thought it was really cool. Especially because it's not appropriate for an employee to ask out a customer, but the other way around is totally ok.
Point is, even if he is interested he can't say anything at work. To be honest, it really doesn't matter how you do it. If he's single, he'll probably call you. If he's not, he'll more than likely still take it as a great compliment and you'll probably make his day. Who knows... even if he is seeing someone, maybe it's not that serious, or it's dwindling.
Seriously though, I think slipping him a note is the best way to do it. He'll have all day at work to think about you, plus it'll bring back to fun little adolescent memories. Worked on me.
there are 10 types of people in this world. Those who understand binary, and those who don't.