Well, you clearly have no intention of budging on your position, OP, but personally, I would not be charging down the complaints/legal advice route. There are other, much less belligerent ways of engaging with the school that would have a much better chance of getting you the outcome that you want.
However, I don't really understand why you are so attached to that outcome, when your ds - who you have said is engaged and intelligent - doesn't seem to care. The "mother knows best" approach is fine when they're tiny, but he is, what, 15? At that age, he is old enough to decide for himself what is important to him, and if he is as intelligent as you say he is, he should be more than capable of making a sensible, informed decision. If he was determined to do triple science and you were helping to fight his corner, then I would say all power to you. Instead, he isn't bothered and you come across as a pushy parent who is overly invested in a particular view of her child's ability that may or may not be reflected in reality.
You seem to have some weird perceptions about combined science. I do a lot of recruitment and rarely see people's GCSE results. Even if they're listed, I don't bother looking at the subjects as they simply aren't relevant. I would make no judgement whatsoever on triple vs combined science. And I say that as someone whose dd did triple and is currently doing science A-levels. Lots of her classmates did combined and they are doing just fine.
But that's all by the by, as you have clearly made up your mind that triple science is the only valid option for someone with your ds's level of intelligence, or at least, what you perceive his level of intelligence to be. Nothing is going to change your mind about that and you clearly believe that the school is doing him a disservice in order to make themselves look good, rather than using their considerable expertise and professional judgement to recommend the most suitable option for your ds on the basis of what they know about him.
The only thing I would caution is that this battle could so easily become a source of extra stress and pressure for your ds in a year when he needs it least. You obviously believe that you are doing the right thing for his future, but you could end up inadvertently causing a lot of damage instead. Please think carefully about how you proceed. Ultimately, even if you're right that he could still excel in triple science, is being right really the most important thing in this situation?