Is Kant correct in thinking that there are exceptionless moral rules ?
There is no way to objectively answer this question. We cannot know what is truly moral because we are simply human beings. However, Kant's arguments do seem to make a great deal of sense. It seems completely appropriate to say that people ought always to be treated as if they are ends in themselves, not simply as means for another person. This idea is the basis of morality in Kant's thinking.
The problem here is that this idea is based on the idea that the individual human being is the most important entity. If we do not think that each individual human being is more important than, for example, their family or the society as a whole, this rule falls apart. We could then require a person to sacrifice him or herself for the good of the society as a whole, regardless of what the person thinks about that idea.
Every moral rule must start from first principles about what is valuable and what is not. These first principles are not ones that can be logically debated. They are matters of faith. Therefore, we cannot say in any logical/objective way whether there are exceptionless moral rules.