At operation you find around 700mls of blood in the abdomen and a grade 4 injury to the spleen, probably caused by one of the lower ribs at the moment of impact.
Unfortunately, despite a quick splenectomy, they are never able to restore an adequate blood pressure, and the boy spirals into acidosis and coagulopathy, none of which does his brain injury any good. He dies on the intensive care unit a few hours later.