[cntgetmedown]

Firstly I agree that there is also value in calling this preflop to keep in the bad player.
I don't see why he can't also have AQ in his range here. If he has some floating range. His range with sixes probably contains 66 and 56s as JamesMa mentioned, though I'm not entirely sure he would defend low suited aces. Anyways if he does than he can have more chops in his range.
I just have trouble putting him on a significant bluffing range. What hands do we think he's floating us with here on the flop that have not improved and will be bluffing here. Do we think he will turn weak made hands into bluffs here, say 88 and 99? Can he play KQs this way?
Something I think we should also consider is do we think he can play and valuebet worse this way? I think a decent amount of players will try to set mine this deep with position. So that puts hands in his range that might be overplaying against our hand, due to perceived strength.
This is minor, but does villains bet sizing appear awkward to you? He could have bet more on the turn to make the river less than a pot sized shove.