How can I get my son to stop wetting the bed at night?

My son is four and the sweetest little boy ever, but he has been having issues with wetting the bed for the last 6 months or so. We make sure he doesn't drink anything before bed, he wears a night diaper now, and we make him use the bathroom before bed too. What else can I do to make sure he doesn't wet the bed? It is not every night, but then he also has accidents at day care when he takes a nap. He doesn't have accidents during the day, only when he is asleep.