From your post of Zimmerman 911 call, leaving the car he is big heavy and out of shape and walking, attempting to keep Trey whereabouts in sight... again there is no law for following.
Here is something else... because it wasn't allowed to be discussed at the trail - Treyvon was casing the neighborhood for a break-in. He had done it before and was expelled from school for having stolen jewelry, as a minor it was withheld.