No, his nephew, the provisional new Duke. If the Duchess isn't pregnant, then the nephew will be heir to everything, no worries, assuming nobody shoots him or blows him up. If she is, however, it all becomes a lot more complicated. Since one of the things he is heir to is the throne, they are all being very careful about it, and if the Duchess is pregnant they will have to work out whether having a newborn king or queen is really a good idea and decide where to go from there.