He is teaching his child a basic martial arts stance when it happens.
Stein believes that he is a good father. He can see himself in his son, in the way the boy withdraws into himself and tugs his own hair until he decides that he wants to eat or take a toy apart or curl up in his father's lap. Stein recognizes that there is value in touching a child that is your own, that there is something special about being responsible for a life that you created. But it isn't until a spring afternoon, after five years of rocking chairs and shoulder rides and learning to raise a child, that it happens.
He and his son are both shirtless—Stein out of habit, the boy because he wants to be like his father. Stein kneels behind his son and gently puts a hand around each of the boy's wrists. His hands dwarf his son's, and he unconsciously ends up cradling them as he adjusts the child's arms into the right position. There is less than an inch of space between them, and when he breathes his son's hair rustles gently. All these factors combine with all the previous years of Stein's experience, but that isn't what causes it. It happens like this:
The son, blindly trusting and enamored of his father, closes the gap between them by subtly leaning into his father's semi-embrace.
The father, struck at once by the combination of the moment's events and a lifetime of searching experience, lets his other knee fall to the ground and wraps his arms around his son in a real embrace, face half-buried in the other's hair. He sheds a few silent tears before he lets go and resumes the lesson.
Later, after his son is asleep, Stein turns his screw around and around as he tries to quantify to himself the feeling that he's always sought but didn't actually find until today. He finds that he can't. He understands, now, why no one could ever make him understand what love meant. It can only be experienced.
The screw stops with a click; Stein walks quietly, but with purpose, to the doorway of his son's bedroom, and finds that it's easier than he would have expected to experience it again.
And he believes that he can live with that.