I think if he is applying for the most selective rigorous boarding schools, any negative is going to work against him. AND perfectionist tendencies can lead to mental health issues in an environment where the vast majority of students were used to being at the top of the class and all of them can no longer be there.
If he is applying for some of the kinder gentler boarding schools, especially the all-boys schools, IME they will be more forgiving in admissions and there is a greater likelihood that he will be able to be at the top of the class as the student body is broader in its academic capabilities.