Larry Keller never actually takes the stage in Arthur Miller's All My Sons. What the audience knows about him is limited to what is conveyed by the other characters. Though he never appears on stage, he still remains an important character in the story.
What happened to Larry during the war is not entirely clear at first. Officially, he is listed as missing in action, something that gives his family hope that he may still be alive. Kate, in particular, looks for signs and omens that Larry still lives and will return someday.
These hopes are dashed towards the end of the play. It turns out that Ann Deever, Larry's former fiancée, knows the truth of Larry's fate. We learn that Larry took his own life during the war. Ann has Larry's suicide note, in which he confesses that he cannot live with the knowledge of his father's crimes. Ann had hoped to keep this secret to herself. However, Kate's stubbornness in refusing to accept that Larry is dead finally forces her to reveal the truth. Only then can Kate accept the reality of her son's death and consent to a marriage between Ann and Chris. It is this new knowledge that leads to Joe's own suicide at the end of the play.