Er, earth magic. Because it's outside of his control, and binds Peter to Narnia despite what Aslan might otherwise wish. He doesn't want Peter to be as connected to Narnia as he is; he knows that he's barely keeping a hold on Peter and if there's something that binds Peter otherwise, Peter will always choose Narnia over Aslan.

Vague plot bunnies aside, this will, I think, be the first fandom I don't go AU in. (You have no idea how tempted I am to AU The Last Battle, though. Peter and Edmund instead of Eustace and Jill! Or Peter and Edmund and Eustace and Jill. *vaguely bemused*)