note- i think its okay to fold our whole range here for 3 reasons

1. he doesnt know we fold our whole range here

2. if his strategy is 'bluff shove for 5x pot 100% of the time with weak hands' or even 10% of the time, we can exploit the hell out of him once our sample increases. if his strategy is just to make a one time play of bluffshoving here knowing our range is weak that's still probably a poor play just because the bet is so huge relative to pot and he can't have perfect information of our tendencies ... meaning his bluff is prob a huge loser vs the population of regulars in the game.

3. if his strategy is 'value shove for 5x pot when i have KKK on a 99x board because he cant fold a nine' he obviously missed a bet and potentially a raise AND he is subtracting a ton of value from the rest of his range as we can extrapolate that his river overbet strategy is to hold the concealed nuts and begin to bluffraise smaller river bets as well as vbet river more aggressively when checked to as well as call smaller river bets with very weak hands.

conclusion: his bet sucks and we should fold and laugh at him