This adds to the theme of how insignificant life is
I feel that Oskar mentions Stephan Hawking's A Brief History of Time to make himself feel tiny and insignificant. The book explains the creation of the universe and emphasizes it's enormous size. Oskar tends to have "heavy boots" when he thinks about how insignificant he is compared to such a vast world. He often feels that he has no point or role on earth (life is meaningless) and feels confused about life itself.
"I held up the sheet of paper, with the first page of A Brief History of Time in Japanese, which I got the translation of from Amazan.co.jp." (190)
insignificance of people and significance
On page 227, Oskar says "that's not really a question you can ask about it." when asked about the book A Brief History of Time. Maybe this is said to emphasize the blankness and how boring Oskar thinks his life is because not only does he not try to make his life better, but he even reads dull things. This seems like a bad idea to me.
After reading the first chapter of A Brief History of Time, Oskar says he "got incredibly heavy boots about how relatively insignificant life is, and how, compared to the universe and compared to time, it didn't even matter if i existed at all. " (86)