If your ds was bright enough to get in, as long as he is hard working, then he will get along just fine. Effort goes a long way. Well done on him for passing the entrance tests.
With regards to a “simple” background, I really wouldn’t worry about this. As with any indie, there are parents who are mega rich. There are also dc whose parents are in normal jobs who can only send their dc there due to gp’s providing a trust. There are lots of parents where both parents work who work hard and can afford the fees. There are also many parents with only one dc who can afford to privately educate due to having an only. Proximity to the city also means that there are quite a few parents who weren’t privately educated but who have good jobs in finance.
What was your impression on the school when you visited? What did you think of the Head? Did your ds like it? If you’re concerned if it’s the right fit, maybe speak to the Head at your current school. I think they would probably have advised you against applying if they didn’t think he’d be happy there. Are many of his friends going there?
Middle of the road dc tend to be the ones that really flourish in an indie due to the smaller class sizes and they can’t hide in the crowd so get the help they need. In a state school, they try bloody hard but it’s easy to be overlooked if you’re not either the brightest or one of the very weakest.
I personally would choose Brentwood over any state schools in the area. However, you need to speak to your ds and decide if he’d be happy there.