I'd talked myself into believing a KJ or QJ hand, but on second thought - that's just improbable. Given his stats he'd have done the PF limp and call, flop call and turn check with a huge range of hands (expecting BB PF raise and cbet not necessarily to represent too much strength). He could be sitting on a middling pair, missed straight, missed flush or even KQ with no problem. The only real thing preventing the river bet from meaning "So you didn't have anything anyway then" is that it's a bit too feeble. Bayes theorem really - very high probability that KJ or QJ would've played in this exact way, but a low probability that a huge range of hands would have ended up here as well greatly outweighs that. You're more likely to be up against middle pairs or missed draws than anything else - probably a hand that's both (like 76s).

Oh, and raise river. I like something less than pot sized which he might call. Maybe raise to $6 or so.