Just try to shame him or something, pestering him will do a great job. Don't say thing directly, use words that has the same meaning when you're around him. Just let him know that you need the money back.
Last time someone owed me 12 dollars, I just went to his back and gave a big slap and asked him for the money, in front of a girl he had a crush on. He can't really do anything but throw a tantrum and throws one or two $2 notes on the floor. I picked it up and went to his face and sort of flicked it at him and stuffed it down the drain. After that, I said "I am not a beggar like you who needs to beg for money and what is worse is that you need people to chase after you. If you're that good, ignore the money and walk away."
He chose to pick it up.
