A5? He woke up after checking the turn and raising the flop. He should have bet the turn with the A, but maybe he put you on an Ace with a better kicker than his 5, but not AK, etc. but not good enough to keep raising the flop. A10, AJ, or something. He hits two pair on the river, figures they are good and goes for it. Could be an A4 for the straight as well.