I think checking is fine. You have nothing so far and the pot is already big. Since you check raised the flop, he should be wary of you doing it again on the turn, so I think he will only bet with premium. If the ace scares him, he should check back by fear of being check raised again (you said he is cautious, right?). If he checks back, lead most rivers. If he bets big, you probably have to fold based on pot odds because he is not going away. If he bets small enough then call and try to make your flush (you can't count kings and queens as clean outs here anymore).