Kids these days will probably read it online.
Given how ignorant Ignorant Girl is? It wouldn't surprise me if their Internet at home is
severely censored.
(I've actually heard stories about people who were raised so strictly that, as adults, they had no idea about how to make babies. They thought just laying together in bed would do the trick, and never knew any better until they went to their doctor about it. Granted, this was before the Internet came to be as widespread as it is now.)
So I'm thinking that he did indeed raid someone's porn stash. Or, more likely than not, came across that one book in a trash pile and has thus far been his only exposure. Which would explain his niche taste at this age.