"He asked me to do it, and so I did it," Robinson told the New York Post this week.
"It's as simple as that. There's no deep dark secret," Robinson said, noting he and Franco met on a plane two years ago. "James is my brother."
Robinson did not say whether or when the song would be released to the public.
Celebrity Families of 2014 [PHOTOS]
S Club 7 to reunite for BBC charity event