Every time I try take the quest "Purity" to heal Vilkas of Lycanthrope he will follow me initially but as soon as I leave Jorrvaskr he will immediately turn around and re-enter, and make me abandon the quest. He is still semi-linked to me, as when I pull out my weapon, he pulls out his. But he won't follow me and the quest is abandoned. I wanted to cure myself in time for Dawnguard so I could become a Vampire Overlord. Please let me know if there's anyway to fix this.