He doesn't. And, FWIW, if Zimmerman's account is correct, that he was returning to his truck and was surprised and attacked, then there is no application of "stand your ground" at all. In order for "stand your ground" to apply, a person needs an opportunity to retreat from physical blows or other use of force; and chooses to stand his ground rather than retreat. Absent an opportunity to retreat, the question about whether or not there is a right to stand ground doesn't arise.
It wouldn't surprise me if Martin attacked Zimmerman for "dissing" and tried to inflict a beating he wouldn't forget, at the end of which Zimmerman pulled out his concealed weapon to avoid being pounded to death.