That is the exact argument he is making!
Based on the fact that he was physically attacked after he had returned to his truck, it is self-defense.
"776.041 Use of force by aggressor.--The justification described in the preceding sections of this chapter is not available to a person who:
(1) Is attempting to commit, committing, or escaping after the commission of, a forcible felony; or (2) Initially provokes the use of force against himself or herself, unless:
(a) Such force is so great that the person reasonably believes that he or she is in imminent danger of death or great bodily harm and that he or she has exhausted every reasonable means to escape such danger other than the use of force which is likely to cause death or great bodily harm to the assailant; or
(b) In good faith, the person withdraws from physical contact with the assailant and indicates clearly to the assailant that he or she desires to withdraw and terminate the use of force, but the assailant continues or resumes the use of force."
Self-defense still applies. If you punch someone in the mouth, they can punch you back. If they start beating your head against a desk (or sidewalk), then they are using lethal force to respond to your non-lethal attack.
Even if Zimmerman 'provoked' Martin by using racial slurs or shoving him, he would retain the right to self-defense if he reasonably believes the force Martin is using is now at lethal or great bodily harm level in response. A punch in the nose isn't lethal force - but pounding a head against concrete can be.
That is also what Zimmerman told police within minutes of shooting Martin. Hard to see how he could have concocted a story that fits all the evidence in that short amount of time.
OTOH the so-called probable cause affidavit relies heavily on the girlfriend in Miami's account, which was not given until several weeks after the shooting and the police tapes and other evidence were made public.