He does know it and he doesn't know it. He feels he should know it now while her hands underline it.

He does need a moment before he can properly lift his own again.

/If there's time./

She's known him for some time now. She's likely known men in his position over the years. It isn't a question of losing the will to keep going.

It's that watching another human being's last breath makes one's own next breath feel completely uncertain.