[Geode]

Raise more on the turn, or even b3b. (If he has nothing he folds to your bet on the turn, but you wouldn't have made much money anyway. But if he raises your turn bet you can play for stacks which is your goal here. If he has A6 or any 6 he probably raises the turn.) Anyway raise more the turn, you are trying to stack him.
But I check the river and call in within reason. He definitely doesn't call with anything weaker, but he might bluff at it and you're happy to call if reasonable. I think a blocking bet works better if the flop was 2 to a flush and you can pretend you were semi bluffing all the time and got there. But he would never believe you got the backdoor flush and might reraise the river repping the flush. If he has a boat you lose either way so don't worry about that.
My guess is he had 77 and donk called the river bet. Just kidding.
Of course my advice could be very wrong, but that's how I would play it.