Sorry that detail threw you out of the fic. I've not watched the anime and the scanlations don't tend to include honorifics, so I assumed that school-aged Yamamoto would use -san with Hibari (if not -senpai), which would be fairly ordinary behavior for a nice boy like Yamamoto. I've since been told that Yamamoto--school-aged!Yamamoto--does just call him plain Hibari, which is an interesting sort of character note that I'm going to have to take away and chew on for a while.