[phish]

Even if you were in a particularly risk-aversive mood, I think you need to call the flop. You can then fold if he bets the turn or win the pot if he checks the turn.
I call these plays 'give your opponent the opportunity to make a mistake.' The mistake in this case being he should bet the turn regardless of whether he has an ace. But if he fails to bet, then in essence he has surrendered the pot to you.