A big raise might have got him off, looks like he was just hoping to see the flop for a buck or 2 to try and flop the flush draw... I think a big raise pre-flop or on the turn could have got him off... a raise of any size on the flop, He calls... and If he calls a big raise pre-flop, Hes calling any raise on the flop, and then that probably makes it worth while to chase on the river