If this was open source work, sure. But he's charging quite an amount for something I personally consider plagiarism.
EDIT: For anyone unaware, he's selling these on his personal site.
2. 'Style' is not copyrightable. No one owes royalties to Picasso for Cubism.
3. Plagiarism is representing someone else's work as your own. The tag line of his site is literally "Tolkien Inspired Cartography". He's very clear that he's following Tolkien's style.
4. Tolkien borrowed from previous cartographers, his own style evolved considerably over time and what you probably think of as the iconic Tolkien map, was actually by Pauline Baynes.
Particularly if their website were called "StarryNightPhotos.com" and they clearly said on their site "these are painted in the style of Van Gogh?"
Because that's all this is. And there are plenty of AI sites that do exactly what I just said but poorly, and probably plenty of artists with a side-job of doing something like that better.
As I love OS maps and Tolkien I really like this - I might even try and get him to do my local map square.