Trivia

Was cast by Orson Welles as Welles' son in Citizen Kane (1941) because of the striking resemblance between the two.

At the time of his death in November 2007, he was the only surviving member of either the cast or crew of Citizen Kane (1941). He portrayed Charles Foster Kane III, the title character's son. Coincidentally, his elder sister Ann Bupp was one of the last surviving cast members of Gone with the Wind (1939).

With the death of Bruce Bennett earlier in 2007, Sonny became the last living male actor to have appeared in a short by The Three Stooges with Curly Howard.

Last known to be married and an executive with the Ford Motor Company in Dearborn, Michigan.

Highest paid male child extra in 1937.

Brother of Ann Bupp, June Bupp, and Tommy Bupp.

Appeared with older brother Tommy Bupp in a total of eleven movies.

In 1941, he appeared in three movies nominated for the Best Picture Oscar: Citizen Kane (1941), Sergeant York (1941) and One Foot in Heaven (1941).

Appeared with Ronald Reagan in the latter's first film, Love Is on the Air (1937).

Interviewed in "Growing Up on the Set: Interviews with 39 Former Child Actors of Classic Film and Television" by Tom Goldrup and Jim Goldrup (McFarland, 2002).