I would watch for the smallest sign of fear or shyness, and respond by placing a loving hand on the child's shoulder, smiling my most understanding and healing smile, and sympathetically explain his behavior as fear or shyness. When he didn't want to pick up toys, he would be confused and fearful. When someone invaded his space and competed for his mother's attention, he would reject them with symptoms of shyness. Just as you could do reverse training on a child, causing him to manifest any symptom of weakness, conversely, you can make a weak child strong within the week. Parents get into ruts that perpetuate negative behavior in their children.
Parents excuse behavior by calling it personality.
Personality is a starting point, not a boundary. Now there are some children that are truly afraid. We are not talking about those who are absolutely terrified of danger, whether real or imagined. That is another matter and is very rare. We are talking about the social response of fear or shyness that causes the child to retreat to his mother's security.