Start slow. Buy a Dog Carrier, or Dog Stroller, one that has a mess cover, and take the dog out in public. If you use a Carrier, I love ours, set the carrier down inside the safety of a shopping cart, even at PetSmart and other Dog Friendly places. Take your dog with you to as many places as you can. Even when your just running to town for short trips.
DO NOT LET ANYONE PET YOUR DOG WHILE OUT AND ABOUT. Just let the dog ride in the COVERED Carrier. Your dog will feel safe inside of the mesh and unthreatened by people. The idea is for the dog to see the world and feel safe.
Little by Little, and I really mean Little by Little. This would be after the dog is very comfortable out and about and napping inside of stores inside the carrier; start letting your dog walk on a leash at Pet and Farm Stores. But still,
hold off on letting people pet him.
Dogs can have a SHY GENE, just like people. Any mammal can. The problem is people think these dogs were abused. Most times they were not. They are just shy and or nervous. Like people, they just need time to process the world around them, rather than being 'thrown into it'.
Some folks will let other people feed their dogs to make the dogs more friendly. This can work.
We do not let others give our dogs food. I do not want my dogs to take food from others. Even at Pet Stores and Farm Stores where the employees want to hand the dogs treats. We take the treat from the person and give it to our dogs. 
Some people would argue that dogs need to eat from others for the fact that sometimes, in our lives, travel, sickness, and other events can happen. We have found that if someone fills our dogs dog bowls with their dogs regular food, no problems. I am talking about people offering the dogs treats.