AK 3 bet gets cold called, wtf river line.

[Carroters]

Villain is 20/17 and doesn't cold call 3 bet lightly from what I've seen. Probably something like 88-QQ preflop, perhaps AQs KQs stuff too. CO opens wide, I 3 bet a bunch. So his range is so so often just 99-QQ on this river, I make a bet size I hope he'll view as bluffy/polarizing like "omg he never has a 5 or a set so he has air" type thing. I get jammed on..

So is he more likely to have flatted some set making lower pair then slow played the flop and turn on this texture...or just plainly have 88 a bunch....or randomly have Tc9c.....or does he absurdly bluff his underpairs anywhere near as often as this, or float the turn with idk AQ to then do this. All of the above seem as unlikely as each other. The only hand that makes any sense is 88 and that's 3 combos.

It's like he can have no hands for value and no hands as a bluff.

Thoughts?

No-Limit Hold'em, $0.50 BB (6 handed) - Hold'em Manager Converter Tool from FlopTurnRiver.com

MP ($27.55)
CO ($50)
Hero (Button) ($63.65)
SB ($63.05)
BB ($25.65)
UTG ($109.05)

Preflop: Hero is Button with A, K
2 folds, CO bets $1.25, Hero raises $4, SB calls $3.75, 2 folds

Flop: ($9.75) 7, 4, 6 (2 players)
SB checks, Hero checks

Turn: ($9.75) K (2 players)
SB checks, Hero bets $6.25, SB calls $6.25

River: ($22.25) 8 (2 players)
SB checks, Hero bets $16, SB raises $52.80 (All-In), Hero?