Sonny was still a son who loved his father, and who was frightened that his father would die. I would imagine that the hesitation was a little insecurity and a little bit of sadness and fear for his father's life.
Ya I always thought that one of Puzo's underlying themes is the fact that the Corleone children were a lot more like the normal, everyday loved kids than say the Tattaglia's or Barzini's. Vito's family was the best family he ever formed.
