Going along with your story, perhaps he did get exposed to the sun and almost perished, and he needed that amount of time to sufficiently heal and regenerate?
Perhaps looking pale and sickly would make sense at first, considering he had been asleep for centuries and hadn't fed in all that time. But maybe his appearance can change as he regains his health.
I think there is a way in-game to turn a vampire into a human, so why not? The interesting would be if this is what he wants, or would it be against his will and he is betrayed yet again.