If he has 77, then he hits a 7 on the turn 1 in 22.5 times. And even then we're not paying him off anything significant. And even then we then have more or less the same odds of hitting river to cooler him.

When he checks back his 77 on the flop, well he's missing 21.5 times for every 1 he hits. That's a lot of potential bluffing spots.

He's gonna bluff 77 more often than he binks it. Same will be true of any smaller pair.

There's no point betting this flop. Our only hope of making money in this hand now is to let him bluff the bottom of his range. We can call one street, maybe two if it's flop and river. I suspect that will show a small profit over a large sample. Betting flop, I guarantee that is -ev. There's just not a lot worse that calls, and nothing better folds.