@Anonuser678543 I'm so sorry to hear about your DS but it's good that he's flagged up his worries with you. That's a great start. Is he at Cambridge? I'm pretty sure that at least 99% of everyone else on his course is, internally, feeling very similar.
Engineering is notoriously incredibly heavy as a subject. DS1 (C) had student friends who were engineers who were studying flat out whilst he was relaxing in cafes and joining lots of societies (he did English) and all of them without exception struggled, particularly in the first year. Two of his friends ended up taking time out and then returning for the following year, as the workload was just so overwhelming.
As far as I know, they give the engineers more work than is realistic to do , especially if, like most students, you're a perfectionist.
Yes, I think it'd be a very good idea that he lets his DoS know how he's feeling and if he has a good DoS, he'll be told that everyone feels like this and not to worry. In fact, if no one has said anything detrimental about his work so far, I expect he's actually doing very well and shouldn't overwork.
No way will be be marked as unsuitable for the university. His academic track record plus the fact that he was chosen in the very competitive assessments to be given a place, indicates they want him and think he can do well there.
If he feels he can't say anything to his DoS, he could also approach his pastoral tutor. He probably just needs some reassurance at this stage and permission not to attend every single lecture or complete every single piece of work perfectly and on time. Sometimes, students are allowed to complete the term's work in the vacation in any case.
My guess is he's nowhere near bottom of the class but he may need to hear than from his DoS, which is another reason why he should let them know how he's feeling.
Meanwhile, he needs a more balanced life even during term-time and it might take him some time to work out how to do enough academics but have space for a personal life and time out too. Overall, to reiterate, I think most of the others will be feeling just like him. He needs to remember how amazing he is just to have got on the course at all.
Good luck to him and to you as well. Worrying about your DCs as a parent in the background seems pretty universal.