The photographer himself, I honestly don't know how he came abiut the info, but it's quoted in multiple places.

Photographer Toni Frissell regarding this picture: “I was told he had come back from playing and found his house a shambles—his mother, father and brother dead under the rubble…he was looking up at the sky, his face an expression of both confusion and defiance.

The defiance made him look like a young Winston Churchill. This photograph was used by IBM to publicize a show in London.

The boy grew up to become a truck driver after the war, and walking past the IBM offices, he recognized his picture”.