Interesting.
I have met several people during my (music) studies and working life who went to Chets, Purcell and Wells. I have to say they were all just pretty "normal" people, with the obvious exception that they were all fantastic musicians. I think one often-overlooked factor is that these schools (maybe excepting the choir schools like Wells) are pretty small. That has certain disadvantages of subject choice, but it also brings with it some advantages in terms of pastoral care. The staff have the opportunity to really nurture each individual and the specialist expertise in what that requires for a budding musician. For a kid like your DC who wants to be doing music all the time and be around other people who are, that can probably be a much healthier environment than being in a normal school.
I can certainly relate to how your DC feels. I grew up in a very non-cultural area and was always "the wierd one" doing music. When I started playing in youth orchestras and later doing a music degree, I was acutely aware of the different assumptions and confidence of the kids who had grown up with it like a natural, assumed part of life. In a field as hard and competitive as music, I think that goes a long way.
As for what happens if he decides not to do music, my impression is that these schools get very good academic results, though not top of the league. Somewhere on the Purcell website is some information about previous A level results, which seem to contain a majority of As and Bs, and destinations of previous leavers which include non-music degrees in several different fields. I think the main problem is likely to be not standards as such, but subject choice. Might be an issue if he leans towards science for example.
On standard required, it's hard to say. They do give guidelines on the website, but these are not black and white and they have ways of looking for potential rather than absolute current ability. The bassoon is a rare and sought-after instrument so that would help. I think most of these places are pretty open about discussing such issues with you, and even letting him come in for an informal "pre-audition" if appropriate, since they understand the difficulties involved.
If he's fanatical about music and has a strong sense that that's what he'll do for a living, I'd say go for it. The advantages outweigh the disadvantages. If not, it's hard to say and would depend on all the circumstances. They accept kids in most year groups so you could always wait a year or two and see.