In a tender moment between Vito and his wife, he might refer to Santino the same way his wife does, as Sonny. I was trying for a moment of tenderness between Vito and his wife.
In the only sentence I could find in the novel where Vito calls him Sonny, it was a tender moment between Vito and Tom about Carlo.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."