My son's new dog loves everyone, except is very afraid of me. It took almost two days for me to be able to pet her. They took her for a hike today and as usual she loved everyone except the men with white hair. They ran across three of these people today.
Did your son adopt the dog from a shelter or someone else or get the dog as a puppy? Sometimes, if dogs have been abused in their life, they can develop a fear of people who resemble their abuser. If there was someone in the dog's previous living condition with white hair who was abusive toward her, that could make her extra wary of anyone with white hair.
Dog's become scared if they had a former owner. Also, it could be someone who met the dog with white hair and the dog got hurt. It will take time and patience. Keep the dog comfortable and stay calm so the dog knows everything is alright. The previous answer from Willa is spot on.