It isn't quite that simple. I never said that Trayvon could use SYG simply because Zimmerman followed him.
Like I said before, if Trayvon did in fact circle around or whatever and approach Zimmerman, then Zimmerman isn't guilty of anything.
But, what if he did follow Trayvon, and confront him somehow? Zimmerman, the wannabe cop, maybe a little too authoritarian in his challenging Trayvon, and Trayvon the kid from the city with an attitude? So, push comes to shove and there's an altercation, and Trayvon winds up dead. In this scenario, Zimmerman had the last chance to avoid it all by not following Trayvon, especially after being discouraged from doing so by 911. Having done that, I do not think he has a case for SYG. I believe the authorities know that too, or wouldn't they have had the SYG hearing by now?