The turn 2 changed nothing. He called on the flop, I really doubt he is folding on the turn with whatever he had, and him just calling tells you nothing. If he raises, you cant call as not only will you likely not have odds to hit your draw, many of your outs may very well be no good, or you may even be drawing dead. You can check behind and take the free card on the turn for several reasons: either hit your outs for free if they are good, give yourself a chance at a cheap showdown if you miss but your current hand is good/holds up, and keep the pot small. I'm not playing a big pot here regardless of whether I hit or not, as the board will be too scary (4-straight 3-flush) for him to pay off with something that I beat. He may pay off a value bet with a set, but would he be flat calling on that board with a set? He would surely raise the turn and lock you out of hitting your outs if he did decide to foolishy slowplay the set there, giving another reason to check turn.