I've just decided on my Geasa.
"If ever a lying word should pass your lips, your final breath is soon to follow."
In other words, he must never say something that isn't true. He can be evasive, vague, or deliberately misleading, but everything he says must be true. For example, if he hears (but doesn't see) a fox run up a tree fleeing from some hunters , when the hunters ask him if the fox was nearby he could answer "I havn't seen it" or "near as the wind, far as the sky", but couldn't say "no" without dooming himself.