The Dennis hearing for 776.032 immunity uses a probability scale of about 50-50. If it's more likely that his action was justified, then Zimmerman is granted immunity. The standard of proof is preponderance of the evidence, same as used in a civil trial.
Most people would not consider that to have sufficient certainty to say "proof of innocence," in the common use of those words.
A declaration by the state, of innocence, is the closest you'll get to proof of innocence. That is different from dropping the charges. I'm thinking about statements made, e.g., when DNA evidence exonerates. Or the Duke rape case, where the accuser and prosecutor were eventually proven wrong.
I've said before, in this case, if there was a movie that corroborated Zimmerman's account, a significant number of people will find him guilty. He got out of his truck.