Beast Boy says that Shazam looks like Elvis. This and the episode's title "Little Elvis" are a homage to music legend Elvis Presley being a big fan of Freddy Freeman, aka Captain Marvel Jr.
"Shazam's" original name is Captain Marvel. Due to expired copyrights, Marvel Comics was able to acquire rights to the name Captain Marvel. For this reason, DC Comics changed the name of the character to Shazam, rather than litigate.
Shazam is referenced as looking like Little Elvis, and ironically enough, Shazam (or Captain Marvel as he was known then) was Elvis' favorite superhero.