Mummytummy, sorry to read about your woes. IMO, it is way too early to predict where a child will be in 10 years time, now. All we can do is guide them the best we can and provide them with the best education we can. Even the very clever DC, mostly get tutored, you are helping your DC. It's also about learning exam techniques. Also sometimes they need to be in the right environment to bring the best out of them. Everyone has a gift, and hopefully with the right schooling environment they discover it, some early, some a bit later. With giving your DS the best opportunities you can, I'm sure he will do very well and you will look back and wonder why you were so stressed about it, when he was only young.
I honestly, don't worry too much about culling, we know it goes on everywhere ( generally speaking), even in some state schools, again it is about engaging the school and making sure your DS, doesn't get lost in the system. If he needs help as GCSEs draw near, then you will do what you have to do when/if that time comes, but chances are, in the right school for him, he will be fine and not need extra help. I think it is more difficult when you have children and one is very academic and one isn't, you compare them ( subconsciously), but each will develop at their own pace.
One of my work colleagues has 2 children and her DS was the same and is now a doctor in surgical training. Her DD, was the very academic one and she continued to do very well and she is an accountant.
Again IMO, art/ music scholarships are just as fantastic ( apart from the fees remission), the schools give extra attention to their scholars ( and others who do well in these fields), and they get exposure and experiences they wouldn't otherwise get, which will stay with them for life and may open doors for them in future which they wouldn't ordinarily have.
Well done to your DD. And all the best with your DS.