"Shazam!" writer Mark Waid explained in an interview with "The Comics Cube" that he always thinks of Shazam as Captain Marvel, regardless of what he's currently known as. Considering Waid is an expert on comic book history and lore and a big fan of the Golden Age of comics, it's no surprise he prefers the old name.
"I'll just call him 'Captain Marvel.' We don't call him 'Captain Marvel' in the book, but I will never not be able to call him 'Captain Marvel' when I'm just relaxed. So in the book, he's just called 'the Captain,' and that is the other kids' name for him," Waid said in the interview. "In the same way that 'the Doctor' sticks when it's Doctor Who. He doesn't go around calling himself 'the Captain,' but when people ask him point-blank, he's got to say something."
While it might be odd to see the hero not be referred to as "Shazam," especially when it's also the title of the comic book he's starring in, Waid seems to want to move on from the current name as much as he can, which makes "the Captain" a solid placeholder replacement. So although Billy Batson isn't on board with being called the Captain, he might not have much choice in the matter. Time will tell if "the Captain" sticks.
"Shazam!" #1 by Waid, Dan Mora, Alejandro Sánchez, and Troy Peteri from DC Comics is available in comic book stores and online retailers now.