In Jerry Ordway's run on Shazam in the 90s he did a tribute to the show. For a brief time the Wizard Shazam took human form and he and Billy traveled around the country in an RV. Clearly putting Shazam in the role of Mentor. It actually makes a lot of sense that Billy and Captain Marvel would call Shazam that sometimes. Saying Shazam's name turns Billy into Captain Marvel. If they said i the real name a lot he would never stay one or the other long.