Did Kant really address why his own metaphysical system is necessary as charting a course for possible human knowledge?  Warnock states that Kant thought:

“All we can establish foundations for is the notion of possible experience and what can be an object of possible experience…”

In other words, physics can tell you all kinds of things about energy, but it can’t tell you what energy is.  And this is the best of our knowledge (which philosophers like Kant try to explain).  Kant’s metaphysics (and religion too) can’t even do that, and are doomed to failure.

In a way, metaphysics may be just where Aristotle left it, or where someone like Roger Penrose leaves it (after a lifetime of applying deep mathematical thinking to physical theory in his work on black holes):  An exercise in trying to develop firm footing for our knowledge after the fact…trying to provide some context for our knowledge and not being able to do so…yet…

Addition:  Of course, this doesn’t nullify the depth of Kant’s contributions, nor the depth of his moral theory.   It just may not make it a moral law in the same sense as Newton’s laws (deeper laws have already come along) .  I dug up one side of this exchange from the Bloggingheads Sentimental Mood Jesse Prinz Edition.  So just because Kant didn’t perhaps validate his project as he’d have hoped, it doesn’t follow that you need to embrace some moral relativism as does Prinz:

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1. If there are facts of a certain sort (chemical, biological, psychological, moral, whatever) which may be true true, even though everyone thinks they are false, then facts of this sort (chemical, biological, etc.) must never change. 

The trouble is that you yourself don’t actually believe this principle. For example, geographical facts are clearly objectively true — even if everyone believes in El Dorado, El Dorado doesn’t exist; and even if no one believed in Everest, Everest still would exist. But it’s obvious that it doesn’t follow that geographical facts don’t change over time. Mountains and polar ice caps and rivers come and go. Geographical facts change all the time — it’s just that our beliefs don’t change them.

Perhaps your point is, not that the geographical facts don’t change, but rather that the geological laws don’t change. But at a certain level even this isn’t true: Plate Tectonics fits the earth over most of its history, but it doesn’t exactly fit the earth when it was molten or when it eventually cools down completely in the distant future. It doesn’t fit the moon or Jupiter. Perhaps at a high level of abstraction, we can imagine a final theory of planetary geography that fits ALL types of planets anywhere in the universe. Perhaps these very abstract laws don’t themselves vary (once there are any planets to talk about).

But if you make the moral law sufficiently abstract, it can be as unvarying as any laws of universal planetology. Utilitarianism is the theory that there is a rather abstract law of morality, which, though it does not vary, accounts for why seemingly quite different things can be right or wrong in different circumstances (e.g., why leaving your elderly to die may have been OK for the Eskimos in conditions of scarcity while it would be very wrong for us).