@Masalamother I completely empathise with your post - my son is in his second year of studying engineering at Cambridge and found it difficult in his first year. He knew it would be hard work, knew that he would no longer be the cleverest, but I think that the immense pressure and the short terms were a shock to him. He also got four A stars in his A levels and is used to working his socks off - but the way that the system works and the way that exam questions are structured and graded, has taken him some time to adjust to. He too was slightly regretting not taking up his Imperial offer, thinking that longer terms may have been better.
Fast forward to now, he's about to finish his second year, and we are in completely different territory - he's realised he's not going to get a first and has made peace with that, and is living life to the full - as much as he can - around his workload.
I know that they HATE taking help or seeking advice from their DoS or tutors, particularly on the academic side because they see it is a 'failure', but that is what they are there for. Perhaps you can encourage your DS to have a chat with someone on the pastoral side, it may help him realise he's actually doing better than he thinks.
And then it's the long vac, and maybe over the break he can reassess how he's feeling and go back refreshed in October. At least they know what to expect in the second and subsequent years - I think it comes at them so fast in the first one! If he's REALLY unhappy and not sure - could he intermit? DS had a couple of friends who did this after their second term, but came back readier in their 'second' year - although this is potentially tricky with funding.