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 is one of the last surviving cast members of Gone with the Wind (1939).
Highest paid male child extra in 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).
Last known to be married and an executive with the Ford Motor Company in Dearborn, Michigan.
Was cast by Orson Welles as Welles' son in Citizen Kane (1941) because of the striking resemblance between the two.
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.