50NL~ TPTK in 3b pot

vil is 29/19, 54% afq, 75% fold to 3b, 2% flop raise over 150 hands. Last time I 3b him he called oop w/ATo


SB: $49.50
BB: $91.51
UTG: $55.50
MP: $68.97
Hero (CO): $70.90
BTN: $55.73

Pre Flop: ($0.75) Hero is CO with A K
1 fold, MP raises to $1.75, Hero raises to $6.00, 3 folds, MP calls $4.25

Flop: ($12.75) A 7 6 (2 players)
MP checks, Hero bets $8.00, MP raises to $20.00, Hero ???


I have no idea what he's doing this with. He's repping such a narrow range here but A7/A6 seem a little out of his flatting range and so do 77/66... TP with a flush draw? Do I flat and keep all his possible bluffs/semibluffs in or make a tight fold? Can't decide which option I hate more