too passive here?

[furfur]

$1000+50 live. 200 entrants. 120ish remain. Blinds 50-100. Hero w/ 8k. Villain with 11k. I've been at table only a couple of orbits but villain has been very active. My image is standard. Folded to hero in CO w/ A,Ko makes it 300. BB calls. Flop is 3,6,9r. Checked back. Turn is 5. Villain bets 300. Hero calls. River is J. Villain bets 500. Hero calls again. My reasoning: 1)villain is most aggressive guy at table, 2)Most (live) players suck at thin value bets. If he's representing something on turn, river couldn't have helped him and he would be very content to check down a mediocre hand against preflop raise from tight player who stuck around after turn. So he either has a monster or air and as he's shown himself to be very aggressive, chance for latter is sufficiently high given pot odds for me to call. Thoughts?