Johnny Sheffield claims that he was handpicked by Johnny Weissmuller after screening hundreds of applicants for the role of boy. Weissmuller is also the person who taught Sheffield to swim for the movie.
In the original script Jane was to have died (Maureen O'Sullivan wanted out of the Tarzan series). Fan protest forced a last-minute rewrite in which Jane recovers from her near-fatal wound, admitting she had been wrong to disobey Tarzan.
It is stated that "Boy" is the heir to the Greystoke title. In the original books, it was Tarzan himself who was the heir to the Greystoke title.
Tarzan and Boy are filmed playing underwater hide-and-seek in Silver Springs, Florida. Halfway through the shenanigans, a medium-sized alligator can be seen quickly swimming past Tarzan and son and disappearing in the background, without anyone appearing to notice it.
Tarzan had to "find" a son, because censors would not allow Jane to become pregnant because she and Tarzan were not legally married.