If you look at his classes you'll see several of them have +1 spell casting, mine does the same. If a template/creature gives casting as per an X-class then you can improve it by taking levels of either that class or another class that improves the casting.<br/><br/>I increased my own druid casting using mystic theurge in the same way with nymph granting druid casting and the theurge granting additional casting levels as well.