If you're talking about Sep 2021 entry, I'm afraid application is now closed - last Friday, 25 Sep. Sixth Form entry via the normal fee-paying route is extremely limited; I can think of numbers less than the fingers on your (one) hand. Places usually arise when existing boys leave the school (very rare) after Yr 11 to go elsewhere, e.g. employment, overseas colleges, etc.
But since you're asking the question now and the nature of your questions, I'm assuming your boy has been selected for assessment. Unlike the normal entry route for 10-year-olds for Yr 9, Sixth Form entry is rather simplified in the sense that the school would have already known much about the hopeful applicants at this stage and age.
Essentially, what Eton is looking for is what, and how much the boy can bring to the school. Although the minimum GCSE requirement is considerably "low" (6 subjects at Grade '7' or equivalent) in reality, the quality of boys applying is much, much higher - which in fact, is what Eton expects.
To be honest, I don't even know if potential applicants are even tested in Maths, Verbal Reasoning, etc, etc. unlike the younger applicants for Yr 11 entry. Like I said, Eton would have already known much about the older boys judging by their GCSE results and other achievements by this stage and therefore, those other preliminary assessments would have been a complete waste of time.
What's more important to Eton now is what the applicant's teacher /headmaster has got to say about the boy? What achievement(s) besides academics has the boy gained? What future plans and ambitions do the boy have? All this would have to be revealed during the INTERVIEW which is what the "assessment" is all about.