The only time you should bet so much less on the river is because you're going to re-raise. The 5 strengthens your hand, if anything - it doesn't add to his straight chances, it doesn't pair his ace or queen - he could have a 5 in hand but it doesn't matter as your wussy bet gives him every excuse he needs to push.

My guess is he had a king and you were always going to be beaten. But the problem is you didn't give him a reason to beat you with cards.