Changing Facial Recognition Name

Photos seems to have a problem to keep the faces of close relatives apart. I would not invest too much work right now into fixing misidentified faces and wait for an update.

You can try to do it this way: https://help.apple.com/iphone/11/#/iph9c7ee918c

Remove a misidentified person. Tap a person in the Photos album, then tap Show More to see all photos they appear in. Tap Select, tap Show Faces, then tap the misidentified face. Tap the Share button,then tap Not This Person.

But several users reported, that Photos keeps merging the same persons over and over again.

one of my Grandson's files completely disappeared?

Try to find one of his photos to add the person album back.

On a photo of someone you want to add, swipe up to see a thumbnail of that person (below People). Tap the thumbnail, tap Add Name, then enter the name, or tap it in the list if you see it (names in the list are from your Contacts app). Tap Next, then tap Done.

If you view a photo of your grandson enlarged, touch it in the middle and swipe the photo up. Then you will see related information about the photo below it. If a face has been detected, there will be a thumbnail.

For example, I swiped this photo upward, and now I can see the faces of the people below. Tap the thumbnail of the face.

What if a face is not detected, can you help it by entering.

No, that is not possible in Photos on an iPhone, only in Photos on a Mac. On the iPhone you can only name faces that have been detected automatically by Photos.