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Fold, there are too many hands a calling station can have here that you are behind. It has been my experience that passive players won't show resistance to an opponent who has been betting at them unless they are holding two pair or better. The only possible hands he could have that you beat are 64 or 94. The way he played it, I see two possibilities: He flopped a set with 44 (or maybe JJ or KK if he's ultra-passive). I have had this happen before when a passive player flops a set and calls the flop and turn and then bets/raises the river. The other possibility is that the river helped his 99 (passive fish hate to fold their pretty pocket pairs, even if they are underpairs to the board) J9, or QT, or maybe K9. Since you raised PF, he might not play back at you on the flop or turn TP/no kicker, but he aint gonna fold it if you bet. He hit his kicker on the river and he knows his hand is good.