Sonny was almost definitly underboss for the old man. After the don is shot even if he is acting boss just for then why would he say "if the old man dies god forbid do we take the deal" obvisley if DOn Vito died his underboss would take over.
I think Sonny became the unofficial acting Don after Vito was shot. In the novel, Vito had his doubts that Sonny had what it took to succeed him, so he never officially appointed Sonny underboss. But after the attempt on Vito's life, he obviously couldn't make decisions for the Family, so Sonny had to do it for him.
