Biography and the cutting-room floor

I have a final chapter still to write of my J. S. Haldane bio, but what the heck. I've paused and stepped back to edit those earlier ones.
Now's the time to see the story flowing, to give the writing itself an extra dash of verve. It also, sadly, means some of the details I was most proud of finding get dropped from the narrative.
As the first-ever bio of J. S. Haldane, the need to present a compelling figure is vital. Were he one of the Virginia Woolf, D. H. Lawrence types of figures, who have a fresh bio as each crop of university graduates grows up, those tiny details would become invaluable. Now they get in the way.
The virtue of the cutting-room floor in biography writing is that many things can drop into the footnotes.
*Here's one such footnote. the photo shows Haldane with his daughter Naomi (tpo grow up into the novelist Naomi Mitchison) on his knee, and son Jack (J. B. S. Haldane) leaning into him. Probably taken on the lawn of their home on St Margaret's Drive, Oxford in about 1901. Who, though, is the young man on the left. Any ideas?