In this sentence, you're using "Father" is his name. It's the same as if someone said, "Aran, when are you leaving?"
*as (not is)
In this sentence, "father" becomes capitalized because it is being used as a direct address. When we use terms like "father," "dad," or any other familial title as a form of direct address, we capitalize them to show respect or to indicate that we are speaking directly to that person. So, in this case, by capitalizing "father," you are indicating that you are directly addressing your father when asking the question.
