If you have a concealed carry license, you don't exactly walk around the neighborhood brandishing your firearm. I'd be willing to bet he didn't want to pull it out and walk around with it because he didn't now if Trayvon had a gun or not, and you're trained to only use it as a last resort. I think Zimmerman was wrong, but I also think he acted in self defense....
He probably continued to follow the kid, after the dispatcher told him now to, because a suspicious person call isn't exactly high priority on the list of calls for police officers. Sad situation all around.