Sorry I did look at the OP - but hadn't seen that.
These are less for WinColl/Eton where my opinion is more like the others here.
but questions you may want to ask about BOARDING.
I ask about him wanting to board because I wanted to be sure that's what was actually wanted.
Fwiw I am an adult who boarded very very happily (interestingly from Scotland to england 30 years ago from young.
My DC are at state day schools and thriving academically and socially though they lack CCF, rowing and weekend trips to museums en masse.
As the DC reach the ages I was at certain points in my school career when I compare my life then to theirs now, they are definitely happier and more secure and less institutionalised than I was. I developed a coping mechanism that was necessary as you had to always be on your guard as you never knew who you may be sharing a dorm with next term.
I have also worked in a non-academic role in a major boarding school in the past 5 years and would still definitely not want my children to board until sixth form.
What responsibilities will they have in the house?
How many other tutors are there responsible for each house as well as the main head of house? How involved are they?
How much free time outside of co-curricular time is there? What do pupils usually do during this time?
How much external use of the facilities is there? This can be a pain as they are
promised so much access to amazing courts/theatre/pool/climbing wall etc and then the commercial team constantly let it out and pupils barely get a look in.
Is there any kind of a buddy system with older pupils when they arrive in y9?
What kind of house activities are there (my friend is house master and prepared an impromptu pre-exeat BBQ last weekend for all 60 of his house) ?
What kind of activities will they be doing which aren't co curricular?
E.g weekend trips are there? _ the geography will impact this to an extent
How much compulsory chapel etc is there?
Hope this helps-