J.G.
great responses so far. One suggestion said to make it a game and see if he can 'aim' and pee in it. On that idea, two other thoughts:
1. Create a big "bullseye" out of construction paper. Put the cup in the middle and tell him he needs to try to get his pee in the bullseye.
2. Is the cup clear? If so, on the outside BOTTOM of the cup tape a boat or something to it so that he can see the boat from where he's standing (make sense?) Tell him to 'sink the boat' or something like that.
What about saying "When Superman goes to the doctor, he has to pee in a cup too?" (or use whatever character he loves - Spongebob, Super Why, etc).