Looks like a set of sevens or threes to me. Besides not raising preflop, you didn't make any glaring errors. I would probably bet less on the turn (around 1/2 pot size) so I can get away from the hand if he's slowplaying a set (which again, it appears he did). Once he moves AI on the turn, there's not much you're beating except maybe QJ or a strange bluff/semibluff, but it's 170 to call and 500+ in the pot, so a call is justifiable.