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
Sir; Rescued APBT
Red Dog's Reconnaissance Man CGC
General Patton Vom Winter Storm