Sit facing him in a booth at a restaurant. Then straighten your leg under the table and use your foot to massage his "parts". At the movies, user your hand.
Wear and skirt and tell him your not wearing underwear, even if it's a lie. This is a harmless type of lie, right?
Start teaching him to read your lips. How? Well, when you 2 aren't playing this game, let's say at home, say something to him by only moving your lips. Then tell him if he can guess what you said, he can have what you said. Then say "f*** me", or "I want to blow you", or "I want to sit on your cock" Say anything, but make it sexual. Make him figure it out before he can have it and when he does, follow through on it immediately. Reward him for good behavior. Do this until he gets good at reading your lips on the first or second try. Once he's good at, then start "saying" stuff in public. And since he's good at reading your lips, he'll guess it straight away, but since he can't have what you've promised. Oh man, that'll throw him for a loop!
I like this so much, I might start doing it! LOL
He might also be better at keeping a poker face and you're doing much better than you think.