are massively oversubscribed.
They are only oversubscribed because most parents do what you do, apply for about 5 schools with the intention of only going to one.
So don't get phased by the numbers, just focus on exam and interview prep.
Like others have said, your doing everything now and these schools aren't stupid, they know a child from a non-academically rigorous state primary, isn't usually as polished at interview as a boy from an elite selective prep school.
He has every chance of gaining a place, and to be blunt, you are correct, having the cash is the major consideration for private school, because if you can't afford to take a 25K a year hit , your DC's suitablity is irrelevant.
I would suggest asking a friend your DC doesn't know well to mock interview him, asking him opinions on things like what he thinks of humans settling on Mars, or the rise of AI, the implications for humanity, If you could solve one problem in the world, what would it be and why? Those sort of questions, and of course, why does he want to come to Alleyns, Dunstans, etc, if he says because my mum said I had to, it wont go down well.
Also mock exams can help