You can enter a name manually.
Pick a picture with your grandfather in it. You may find one that has a circle around him that says "unnamed." That means that Photos has recognized this as a face, but it doesn't know who it is. In that case, you can enter his name, and Photos will use his likeness to help identify him in other pictures. However, if it's not full face on and not large enough to analyze, Photos may never find a face in the picture.
If Photos hasn't found of face, you can add him manually. With the Info Window open, you can see a circle with a + sign in it.
When you click on that plus sign, a circle will an appear on the picture. You may have to click in the circle or on the picture to let Photos know that you're now focused on that window, and you can move the circle to the person of interest.
Then you can type in your grandfather's name, or "Prince of Wales" or, if it changes later, "King Charles III."
You might find this helpful:
Find and identify photos of people and pets in Photos on Mac - Apple Support
Finding faces is a background operation for Photos-- it doesn't want to slow down your work if you are using Photos. So Apple recommends running the computer plugged in at night with Photos open but in the background. It can take awhile for it to find new faces...