This is a difficult one OP. I went to Oxford and while I got through it, I don’t think I emerged unscathed. I was very different to your son, not so conscientious by any means. My friend who had been head girl at her school and was very keen to complete everything perfectly did drop out and did really well elsewhere.
Maybe things have changed a lot, but in my experience, at least of a non science subject, you have to let a lot go. You cannot do all the reading. It is not possible or advisable to attempt all of it, and one of the key skills you develop is bluffing. Sorry, but this is true. Many of us used to joke that this is why employers would like Oxford graduates, because in so many jobs you have to sound informed and confident when actually you are flying by the seat of your pants. You cannot attempt to make every essay good, you have to hand in some ‘fillers’.
It is a hard adjustment because while I had loved my subject at A level, the very thing I had found most rewarding was in depth study and real understanding of a topic, and I actually found Oxford denied me that. It was better later on in the course when I was able to choose a specialist topic, but at least at first you are asked to cover so much ground that you have to accept a much more shallow performance that feels quite fake.
Also generally, no tutor will tell you if you are doing well. They may tell you if you aren’t, otherwise they will leave you to get on with it. Often your time is almost completely unstructured and the scope to go wrong with self care and organisation is vast.
It is normal to have essay crises and be powering through the night fairly regularly I think. Normal also to have doubts and panics, but what worries me is your son’s work ethic. Depending on how your son copes, it may not be worth it. You can emerge with a first, that’s all well and good, but then what? I have seen it first hand and learned the hard way myself, that grades aside the people who are set for success and financial reward are the ones who emerge with their health intact, not the ones who get the top degrees. There are people who can do both, I wasn’t one of them and it sounds like your son may not be.
I think you should encourage him to talk to any sympathetic and approachable college staff, and other students especially in senior years. Maybe he will get a ‘reality check’ on what is feasible. It’s a marathon, not a sprint. He can’t always be at his best. Also make sure that he knows he can come home whenever he needs to, and that other options exist. This may be teething troubles, but monitor very closely and be ready to accept things quickly if it is more than that.