I was all set to explain what I thought really happened, but after thinking about it I'm not sure I understood it all either. Did the boy and the Slow King trade places, or were they one and the same individual all along? My initial reading was that they were the same person, and the modern boy was pulled backward in time and became the Slow King. But the fact that he showed up just as the ceremony was commencing, while the Slow King had already been interacting with the villagers doesn't really fit with that theory.
That element of ambiguity could be intentional, of course.
P.S. For some reason the short story "The Very Slow Time Machine" by Ian Watson popped into my head while typing this comment. It's the type of story that would probably work on both Pseudopod and Escape Pod. It's not amazing, but it's interesting. Worth a read if you come across it.