The moment after the boy begins to fall, he might realize the inevitability of his imminent death. That point, while he is still in mid-air and has only begun to fall, might be a logical place to break off.
In most modernist novels, as well as in contemporary commercial fiction, nothing is included that does not somehow advance the story. Corollaries include Chekhov's rule that if there is a gun in the first act of a play, it must go off by the final act. In other words, the gun advances the story in the first act by establishing its existence so that it can get used in the final act.
In post-modern literary fiction, all sorts of things can get thrown into a novel that don't have obvious purposes in the telling of the story. David Foster Wallace's novels spend pages describing the contents of rooms, and perhaps they represent a broader post-modern concept of creating a scrapbook of the era, usually revealing the absurd, dystopic nature of our society.
Perhaps long quasi-gratuitous descriptive passages serve to enrich the ambiance of the novel: magical realism pulls off its miracles by tightly weaving myriad details into a narrative fabric then slipping in that one mystical thread that makes the one otherwise implausible magical event seem so real.
Reaching back to the Romantic period, Moby Dick includes lengthy chapters on the technical details of whaling. Abridged editions of the book tend to omit these chapters. Yet they serve multiple purposes. The technical details heighten the credibility of the book.
A second purpose served the the technical chapters in Moby Dick lies in the voice of the narrator purveying themes. My high school English teacher told us that Ishmael ("...whenever it is a damp, drizzly November in my soul...") disappears after the first few chapters of the book and doesn't reappear until the final chapters describing the encounter with the Great White Whale and the aftermath ("And I only am escaped alone to tell thee," the narrative quotes not Jonah but Job as Ishmael floats on Queequeg's empty coffin). Now that I am a mature reader, I have discerned that Ishmael does not disappear at all: he stops talking about his own adventure, certainly, but his voice continues and focuses on the principals of the crew. His bright spark of curiosity and his bemusement at the folly of the human race, which are themes of the novel, are constants throughout the book, including the technical chapters on whaling. So those passages on the stripping down of a whale carcass and the boiling down of the oil are subtly nuanced with the voice of Ishmael to shade the technical details with bonhomie and bemusement.
If and only if your narrative voice is charged with an attitude that rests with your book's central theme, then any passage of description might be an opportunity to exercise that voice, even as it speaks of the messy carcass of a boy who has just plummeted to his death. (I saw people jumping from the towers on the International Version of CNN on 9/11, and it's extremely disturbing, and your book, if you go that far, should be no less disturbing, which is probably a strong reason for not going there at all.)
So if it's sufficient to say that the boy fell in such a way that his death was inevitable, so be it. But if you include descriptions of the body after the fall, there must be some reason:
- Advance the story
- Create a scrapbook of the era
- Enrich the ambiance of the novel
- Heighten credibility by providing technical details
- Voice of the narrator purveying themes
- Or some other motive that furthers your novel's purpose