Very often guys in the CO or on the button will call with the intention of betting the flop hard when the original raiser is in MP or LP. I say you should push. Maybe he has a flush draw, maybe not, but you have the A of diamonds so he knows his draw is not to the immortal nuts. I would think that if he had a pair of kings he would've reraised pre-flop. If he calle with 66 or 33, then you're going to have to pay him off.