The cause of the differences in this case is that not all manuscripts have 'the Son' in them and the KJV was using one of those. Sometimes differences in manuscripts seems simply a copyist mistake based on the similarity of given Hebrew or Greek words, but that does not seem to be the case here. What we have here in my view is a typical well intentioned yet foolish mistake in trying to protect the divinity of Christ.
I imagine the foolish copyist though Jesus must know everything since He was God, so the words were erased. This is foolish however because on many occasions the human nature of Christ did not know many things, while we know that his divine nature always knows all. His human nature is not omnipresent either, nor is it immutable, for it grew and changed. In any case even removing the words still includes Jesus as not knowing the hour because no man knows the hour and Jesus was not only God but also a man.
In other words, the KJV is not correct in this instance and the other majority of translations have probably relied on more accurate manuscripts in this instance. However whenever a difference exists their will remain some debate. Luckily there a not enough differences among manuscripts to bother the understanding that we are able to obtain from them all. It is always this kind of minor stuff that can be explained better from other passages.