Problem is, he actually might have a little something and you won't scare him out of the pot. Moreover, he might be using PA that shows your mucked cards immediatly and then he can see, that you badly tried to bluff him into something. Latest point to stop here was after he called the turn bet, IMO. He didn't believe you. That's just it. So give it up early enough. Why should he fold to the river bet after calling so much? That's the way calling-stations are likely to think.

Regards.