I would NOT use a prong in this case for a correction. There is another thread right now about someone doing this for alertness to strangers and their dog developed reactivity/aggression because of it. If he is already shy and/or previously fearful, you don't want to give him a reason to think that people are bad.
I would do LAT (look at that) training with him in this case to change the association. He turns around to look, you mark/treat. Because really, he is being good. He's aware, but NOT reacting, which you are going to start seeing if you keep telling him he's bad for doing it. Mark/treat each time he turns around. Marking it is going to turn him back to you for a reward. People will start becoming his cue to look at you and you can reward him for looking back at that point, and do a lot of rewarding afterward/before he turns around as well, once his attention is forward.
Danielle, owner of...
Fraggle Rock Collars
Fraggle Vom Richcreek CGC CA
Red Dog's Reconnaissance Man CGC
General Patton Vom Winter Storm