hmmmm.. well, he bet into 2 people on the river so it's definitely not a bluff.. maybe he had a small pocket pair and thinks it's good cuz nobody has really shown strength yet.. or he hit a higher full house, but wouldn't he check the full house on the turn if he made it? I'd call based on your perception of the villain.. kind of weird play by him.. let's say he has top pair on the flop, even bad players would bet that more.. then full house on the turn, I don't see how anyone bets that with 2 players behind