There is much to unpack in your initial post, OP.
If DS didn’t know so much about his friends’ workload and interim results, would he still be unhappy? Is he genuinely unsuited to the workload, which does happen and can be dealt with? Or is he having a hard time with the comparisons to Durham and Nottingham and perhaps with now being a more average member of his cohort?
The latter is experienced by a large number of Y1 Oxbridge students. Some are actually struggling academically. Possibly your DS is amongst them - possibly he could learn to ‘work smarter’. A chat with his Personal Tutor or someone else might be very helpful. All academics are used to this and want to help; several recent threads have featured contributions from Oxbridge academics about the many resources they can point student to.
As an academic and former admissions tutor I agree that a 2.1 from Oxbridge is worth more than a First from most places, including the universities named in your OP. Even with blind recruiting, Oxbridge candidates do disproportionately well and it’s down to both their inherent abilities and, more importantly IMO, their training.
However genuinely thriving at university is important. No place, and this includes Oxbridge, is right for all who are admitted. You and DS need to figure out whether he is having a wobble because of ego and/or external factors, or whether he is genuinely unsuited to the working rhythms of the place.
I wouldn’t turn my back on this opportunity lightly but if there is a genuine misfit I wouldn’t hesitate to transfer. DS will have loads of options!
The social and recreational side is also very important. I have saved it for last because of the possibility that DS is overly involved with his school friends and hasn’t given Cambridge life a fair chance yet. If that is true, and I hope I am wrong, it is surely contributing to his unhappiness. It takes a while to find your tribe!
Best wishes to DS