My DS said the same about Cambridge engineering. His girlfriend spends a lot of time with him but does a science subject, and agrees that his workload is huge. They've said a couple of times that it seems to be more than (or at least very comparable to) the medics.
OP, you say he's a prefect but doesn't do any other extracurricular stuff. What about supercurricular? Does he do any programming or engineering type stuff in his own time for fun? You really do need to be very dedicated to your subject, I think, for Oxbridge, really passionate about it, and for that to come across at interview. DS had done loads of subject-related stuff of his own accord in his free time at home.
In contrast my younger DS is hoping to do computer science at uni this September. He is bright and hoping for maybe 2 As and an A star at A-level if he's lucky but it doesn't come as naturally to him as DS1. He spends most of his free time online gaming with his mates, going out socialising, playing football etc, he doesn't show much urge to go and ferret around online learning new programming languages, researching his areas of interest or entering competitions, staying up all night for one of them in DS1's case) DS2 just doesn't have the same drive in him as DS1 did, at the moment, and he'd be the first to say it. He works quite hard when has to but he would agree with me that he isn't Oxbridge material, even if he had 3A star predictions (which he doesn't), and he didn't want to take Further Maths. He just wouldn't want to put the required work in if he was able to get there.
Which is fine. And normal! He is young, the drive about pushing himself for careers will come in a couple of years, I'm sure. I would be made up if he gets into the unis he's applied to.
Anyway, your son would need to look at the work ethic needed for Oxbridge if he was to consider it. It really is very different from other unis in England, with the exception of Imperial College maybe, I guess? And to thrive there doing a subject like that it's really not enough to be very bright, you have to be super bright. The pace is REALLY fast. A couple of DS1's friends from school are doing engineering elsewhere at very reputable unis and after speaking with them, DS1 reckons he covered the same material in just his first term at Cambridge that his friends took their whole first year to do. DS1 thinks just about everyone on his course has 4 A stars and most have further maths. Sorry, I'm not sure what the equivalent is in Scotland, but basically you'd probably need top grades in all subjects?
If my DS1 sounds like your son, then I'd tell him to really consider applying. If he sounds more like my DS2 then maybe he'd be better off elsewhere.