Hemingway definitely knew what it was like to have doubts, and he wouldn't deny that he'd made a million mistakes with his kids along the way. But he also knew what a truly bad parent looked like.
"That's not going to happen," he said quite definitely regarding Clint. He didn't elaborate that he doubted Clint had the balls even if he ever had the inclination. He didn't add that he thought Clint wasn't even invested enough for that. And he didn't say that he thought Clint would be a terribly smothering and restrictive father, no matter how much experience he had. He was the kids biological father, after all.
"No one really knows what they're doing. But you worry, you care deeply, you love him like nothing else- and God, that boy loves his mama," he assured her. "No one's taking him from you," he told her, and then just hugged her tightly to his chest. "You're a great mom, I promise," he told her, pressing a few kisses wherever he could reach.