We know it is because he has a father with an Mech engineering degree and MSC Masters who got in to a red brick uni with BTechs and who has a successful STEM career now. One of his grandfathers was a very successful aeroplane engineer and the other an electrical engineer.
I'm not saying your son is more or less privileged than others, but he absolutely does have advantages over many other candidates for engineering. He's looking at being a third generation engineer.
I'm not sure why you think that it's all the private school students pushing up the grades - private school students are ultimately a small proportion of all students at A-level. Yes they are privileged but so is your son - he's has the connections and background to help him understand what engineering is. He's looking at being a third generation engineer. How can you not see how much of an advantage that gives him in this field? Far easier access to work experience and a family that understands what he needs to be doing (and when) to be successful. I looked at engineering when I was doing a-levels and I didn't even know what the difference between the engineering types were, let alone which would be good to focus on given my skill set. In comparison, I grew up in a family of lawyers and I absolutely understood similar differences between the legal profession and would have walked into work experience, had I wanted to go down that route.
What Trake is saying is that her son doesn't have easy access to the connections your son has. Her son has privilege through being privately educated but that doesn't mean he's starting from 100 and your child is starting from 10. It's far more complex than that.
Your son will be eligible for contextual offers if his school/background justifies it. If it doesn't, you need to accept that the reality is that this year 39% of state school a-levels were at A/A*. Your son's grades are good but they are simply not good enough the courses that are most in demand. It's only 'unfair' in the same way that it's 'unfair' that a student who is only going to get BBB doesn't have the same options your son does.