Sorry @Masalamother only read about half the thread, but would like to chime in to agree with many - the experience of being the best at your school and then at uni being very middling is salutary.
I didn't go to Cambridge but I went to Bristol in the 1980s and it was tough to get in (I mean it still is). I was easily the best at my subject, in fact lots of my subjects, at my grammar school; but at uni wow was I low down the order. There were a lot of well off people and also a lot of people who were very smart.
I vividly recall a chat in a seminar about what people had done in the summer - they had been to Germany, spent time as interns in some fancy government role and the like - meanwhile I had spent 10 weeks packing bulbs at a factory. And I couldn't even recall the German word for bulbs!
I recall this now and how I cringed, but in fact I am pretty proud of myself that I supported myself with my non-glamourous summer job. And as I say, the lesson that I wasn't the smartest was salutary and one I will never forget, even many many years later. I think you can always spot if someone smart has never learned that lesson.
So that might be healthy for your DS. And bearing it in mind might give him the impetus to stay and do what (after all) he was presumably very keen to do whe he applied.
But equally, as others say, he could restart somewhere else and there is no shame in that. But make sure he does so with his eyes open.