I totally disagree about the pressure being self-imposed, the workload can be insane
I did an arts subject and my friends doing similar subjects at very good universities (Durham, UEA, etc) did about 2-3 essays per term. Most terms I had to do 2 essays a week, and translations and language prep on top. The tutors were very old fashioned and seemingly not used to having state school students around and it felt like we were often left to flounder. The stress of it all honestly made me ill and I was by no means the only one.
In my honest opinion, it’s a very sink or swim environment. Robust people with good mental health who can withstand the pressure will get a LOT from it and come out stronger. People who are a little more vulnerable for whatever reason can often struggle and the support (when I was there) just wasn’t there.
I would agree with the above.
I turned up and we were sat down by the senior tutor who told us "You all think you're clever - you're not". My DoS told me if I was awake and not working for more than an hour a day I wasn't working hard enough. The stress made me really pretty unwell.
BUT - I learnt a huge amount, I made great friends, and I refused to allow myself to be cowed by it in spite of how ill it made me. I went to dinners, I rowed, I did tech stuff for events, I was in a musical, I did college "politics". And a hell of a lot of work - and learnt a huge amount.
I really wasn't a well lady through my degree, and I did struggle a lot with the lack of adjustments to be able to cope with it: no online lecture notes etc, but I think hopefully the pandemic will have changed all that.
I enjoy my degree a lot more in the rose tinted light of the retrospective than I did at the time. But at the same time, I know now I really can do pretty much anything. I've been a hell of a lot more stressed than during my degree in bits of my job I have done during Covid, but I have been able to cope better as well, I think because of it - and because it taught me (after the fact primarily) to really make sure I put sensible boundaries in place.
I think the other thing which is unusual about Cambridge is that it really is for life - I can still go for dinner a few times a year, I can stop in college any time I'm in town and sit down, leave bags in the plodge etc.
This is a very long ramble which doesn't really give you an answer - but I don't think it's unreasonable of your son to worry. My brother refused to apply after he saw what it did to me, applied to various well regarded but not hugely pressured unis and then proceeded to regularly score in the 90s on exams while having time to party. But I'm still not sure if I would choose not to go if I could have my time again, knowing what I know now.