Final answer:
The correct pronoun to use in the sentence "His brother has always been more athletic than he." is 'he' because it's the subject form, which should be used grammatically in this instance for comparisons using 'than'.
Step-by-step explanation:
The correct sentence should be "His brother has always been more athletic than he.", where "he" is used as a subject pronoun. In English grammar, when we have a comparison using 'than' or 'as', we should use subject pronouns like 'I', 'he', 'she', etc., when the pronoun is followed by a verb, even if the verb is not explicitly included in the sentence. Here, the implied verb is "has been", as in "His brother has always been more athletic than he has been". It is a common mistake to use object pronouns, such as 'him', in this context.
Understanding and applying proper grammar rules is crucial for clear communication. In the provided context, the speaker is comparing his athletic ability to that of his brother, which requires the use of the correct pronoun to convey the intended meaning accurately.