There are a few obvious differences: the sentences are longer in Boy, while Tomorrow uses fragments and repetition.
Another difference is in the way Roald Dahl uses adjective groups. Unlike Marsden, who in Tomorrow is all urgent business, in Boy Dahl wants to lovingly describe details from his childhood, including details about the kind of desk he sat at in Prep school.
For example, let's look at the first sentence more closely. How is it structured?