Have you multi-classed him?
I know I've often had bugs with multi-classing Druid/Cleric whom auto-learn spells with classes like Ranger/Bard/Sorc/Wiz who pick spells to learn whereby selecting a spell from the Druid/Cleric spell list from these other classes and putting them onto the action bar will have the game default to trying to cast the Druid/Cleric versions that I hadn't prepared.
