The Dragon and the Emerald King by Amanda Meuwissen

When the last dragon in all the kingdoms, Pax, wakes from his centuries-long slumber, he seeks adventure again—by finding a worthy mortal to gift three scales which will grant three wishes. When the wishes are completed, he plans to move on and never return to the Gemstone Kingdoms. For now, he craves experience—and a new partner in his bed.

He chooses Bertram, the recently crowned Emerald King.
It’s been thirty years since the Gemstone Kingdoms entered a new golden age, but the peace proves fragile. With opponents of magic reappearing, Bertram doesn’t know what to do… except to wish that he knew what the future might hold if he never became king.