Heu... why is it "a nice dukedom"? It is in fact a chunk of land, sparsely populated, bordering frosty Lappland (Santa Claus on his sleigh ting-a-ling) without any royal history and it has never been a fiefdom.
To me it looks the King has not done his very best to give a prestigious dukedom to his grandson... He could have chosen the more prestigious and more historic dukedoms of Södermanland, Småland, Skåne, Halland, etc. Instead he just created a new one from scratch.
Seeing the choices made, I have the idea the King will create an eventual son of Prince Carl Philip and his spouse Sofia Duke of Halland, becoming the 10th Duke in that prestigious fiefdom, also taking in mind that Carl Philip has been the principal heir Prince Bertil, the last Duke of Halland.