Does Shelly Kagan's Moral Theory Depend upon God?

If you watched the debate in the previous post, you know that Professor Shelly Kagan is fond of the theory that our morality is a putative contract between us that we would make if we were perfectly rational beings. The theistic theory of morality is that it is the commands of a perfectly rational being. Kagan's view is dependent upon the fictional existence of perfectly rational beings (or at least one such being). It seems to me that Kagan is, in effect, saying that morality is based upon our pretending that at least one god-like being exists. In other words, morality is what God would command, if God existed. It's not clear to me that Kagan can easily deny that the nature of morality does not depend upon God. It seems to at least depend upon his hypothetical existence. And I think the question could be raised, can an actual existence -- morality -- depend upon a hypothetical existence -- God?