I don't think this is an unreasonable worry at all! As lots of people have said, Saltburn isn't real, they massively exaggerated the ratio, but of course Oxford has plenty of posh twats at undergraduate level. Of course it has. As does Cambridge. As are several other unis like Durham, Edinburgh, St Andrews, Bristol.
But they do not rule. There are tons of other people and you find your friends, you really do. And you get to observe these strange, super-posh people in the wild, and it's educational. It gives you an understanding of how institutions are so badly run. It gives you such a healthy contempt for the establishment. You learn a little swagger.
In amongst all that there are some lovely, posh, privately-educated people who also don't fit into the category depicted in Saltburn. People are people, really, and those caricatures exist but are an absolute extreme.
I found going to a university with a higher-than-average ratio of private school sorts (as a working class girl from a single parent family, first in the family to go so absolutely no experience) was really instructive. Not always pleasant. There were moments. I was sometimes out of my depth in tutorials. There was a little sneering. But I had my friends and it was all part of life and I wouldn't spend any more time worrying about your son because he's going to get a lot out of it, regardless of whether some floppy-fringed lounging Tarquin looks down on him one time.