Which country? Did he ask the travel party for a letter and was such shown?

“Where is your daddy” or “who is this (older person)” are the kind of questions they sometimes ask when a child is traveling with just a male adult who doesn’t fit the immigration agent’s stereotypes of how families are. Even if with a letter that is notarized.

I would hope the immigration agent’s questions weren’t sexist and ageist, but I know the questions from border control types can be deeply rooted in the agents’ prejudices/held stereotypes. Having and using an approval letter doesn’t necessarily resolve the issues when encountering agents’ prejudices in action, but at times such a letter may help and at least get the agents to move on to something and someone else.