[mjtandy]

What do you think he has been calling with?
The King of spades on the nut flush draw out of position and against the odds - he will surely fold and unlikely unless he is a terrible player.
A set that has improved to a boat is also unlikely that he will be check calling with this hand with the danger of another spade falling on the board, a flopped 2 pair such as AJ is also unlikely due to the lack of protection.
An ace with a semi-quality kicker that is a spade, which has improved to 2 pair on the river. This is probably the most likely option, with maybe an AQ or a possible AK, A10. In this situation do you think he will fold his Aces up against an AI river bet? You have represnted a very strong hand the whole way through right from the flop, and your betting indicates you want him all-in on the river.
I would go for it in this situation. Although there is always the chance that he flopped a flush and was waiting to re-raise the river but was scared by the board pairing. Even so i would still give it a shot.