You can delete any shared album you create yourself and add and remove subscribers. But the family shared album is special: it has all the members of your Family Sharing group added; you can’t add or delete members; and you can’t delete the album.
The only thing you can do is to select all photos in the album at once and delete them all.
Or disable the Shared Albums completely. If you disable "Photos > Preferences > iCloud > Shared Albums", the Family Shared album will be hidden in your Photos Library.But you will no longer be able to use Shared albums at all.
Or ask your son to be considerate and to refrain from posting so many photos.