Here's why:
* "He" is a pronoun, and pronouns can be subjects. So, "he" could be the subject of a sentence like "He went to the store."
* Complete subject includes the subject and any words modifying it. For example, "The tall, handsome man" is the complete subject in the sentence "The tall, handsome man walked down the street."
So, "he" can be the complete subject only if it's the only word acting as the subject.
Examples:
* He is a good student. (He is the complete subject)
* The man is a good student. (The man is the complete subject)
* He, the man in the blue shirt, is a good student. (He, the man in the blue shirt is the complete subject)
Let me know if you have a specific sentence you'd like me to analyze!