i don't have the material on hand to deal with this properly, but my admittedly vague recollection is that he considers it a genuine argument for the existence of god, not an argument for acting as if that god exists (which would be counter to his whole point that you act out of respect for the rule and not out of fear of god!!)
it's essentially an argument from morality and a thoroughly a priori one.
while i don't have the primaries here, i did find the following at the stanford encyclopedia of philosophy: --- Kant's "moral argument" rests upon a set of claims about the relationship between a person's leading of a virtuous moral life and the satisfaction of that person's desire for happiness. Central to these claims is the specification that Kant gives to the notion of "the highest good" as the proper object for the moral ("practical") use of human reason. Within the context of the moral argument, the "practical use of reason" consists in the exercise of our will to choose actions in view of — and solely in view of — their moral rightness. In Kant's technical terminology, in such a choice we will our actions on the basis of a "categorical imperative." The "highest good" consists in a proper proportioning of happiness to accord with the measure of the virtue each person acquires in willing right moral actions. The highest good thus includes a harmonious proper proportioning of happiness to virtue for all moral agents. For the highest good to be the object of the practical use of reason means that the actions that I will to be moral actions — i.e., actions chosen on the basis of following the categorical imperative — must also be actions that will effect a proper proportioning of happiness to virtue not merely for myself but for all moral agents. ---
http://plato.stanford.edu/entries/kant-religion/#3.2
j -- http://www.brainmortage.com/ Among medieval and modern philosophers, anxious to establish the religious significance of God, an unfortunate habit has prevailed of paying to Him metaphysical compliments.
- Alfred North Whitehead