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.
We don't have the prong on him anymore, we only use it for training at home now and even then, not often. He is not aggressive at all, he avoids. If he is put in a situation where he cannot avoid, he deals with it and relaxes after a few minutes.
He knows look at me, he does it fine at home, in high distraction areas, etc but will not do it when someone is behind him. If you are suggestion that we mark the unwanted behavior and treat for that - we did that with him jumping up and it worked well. He doesn't jump up anymore unless we say "Bruce, Hugs". The problem with marking and treating him when he's turning around to see whomever is behind him is that he won't take treats when other people are around - it took 3 months of the guy at the hot dog stand offering him pieces before he'd even take one, he won't take treats from the people at the beer store and we're there pretty often, he knows the girls that work there, he won't take them from pet stores either. It also doesn't help if I have the treat and try to give it to him, he won't take it then either. I've tried taking his ball on walks with us and getting him to focus on it because at home he would do anything for it - but nope, not even the slightest glance from it. I don't know how to mark it for him when there doesn't seem to be anything that I can treat with.
I realize that part of it is an exercise issue - it has gotten worse over the winter since we haven't been able to have him out as much. He's going a bit stir crazy at home and acting like a wild beast if you don't tell him what to do. I do OB with him for 20 minutes before I go to work in the morning, usually on my lunch break and then in the evening we'll do about 45 minutes to an hour of OB mixed with play. BF also does a bit of OB throughout the day. I walk him on my lunch when I have time (if my break is only an hour we just do OB, most often it's 2 hours) and in the evening. If BF is going out during the day he usually brings Bru with him, so that's another walk. During the winter, he's just had the OB and one walk/day - the weather was exceptionally cold on and off over the past couple of weeks so it has been less.
I guess in short - we don't use a prong to correct him, he won't take any type of treat to reward the mark with.