Under Gorse and Stone by Lily Morton

Cary Sutton isn’t surprised when his boyfriend walks out on him during a holiday in Cornwall at Christmas. It’s far from the first time that someone’s accused him of being cold—both inside and out. Cary’s spent his whole life feeling like he’s separated from the world by a sheet of glass. Nothing touches him. Nothing moves him.
Which is why it’s so strange to find himself drawn to a small Cornish beach and the man he meets there. Sigurd Arvesen is handsome and charming, but he also holds a secret—one that’s big enough to overturn Cary’s whole world.
Sigurd is a centuries-old dragon, and Cary is his mate.
Whisked into Sigurd’s magical world, Cary meets strange and wonderful creatures that he’s only ever read about in books. The mystical realm is both enchanting and dark, but even scarier are the warm feelings that Sigurd brings out in Cary.
What will happen when Christmas Eve dawns and Cary must go home?