the state can still move forward and prosecute him. If they feel they have enough evidence. Diane already said it was self defense which is an admission that did in fact deliver the beating. And I’m sure the police photographed his bruised/swollen knuckles:
Lastly, Cyrus told Laura and Kevin that Sonny is the one who attacked him. So even if Nina got him to decline to press charges—they could still go after him easily enough.
But then again just as Carly is the Teflon Queen. Sonny is the Teflon King so I don’t have high hopes of a conviction.