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. 
