Wow @NCworries I agree that is a big drop and I would be concerned about it.
I wonder if it is possible to find out from the uni roughly where he sits in the cohort? As a pp says, he probs is not the only candidate accepted with CDD. Gosh tho. DD had RG offers of ABB and got BCC and no chance of either of her offers or any other RG unis accepting her (this was in 2019). It turned out OK in the end tho and in fact where she went (Leicester) was good in lots of ways. Anyway.
I also agree - why did his grades drop so much? Assume his PG was ABB or similar? Two grades per subject is a big drop. Does he struggle with exams? Will the uni course be exams or assessment (DD's was the latter and she shone there, like @Cinaferna's DC she avoided exams)? Or is there poor revision technique? Were his GCSEs better than this? Was there some personal issue (breakup with partner, family issue) that meant he did worse than expected? If it's some reason like that then that may be less of an issue for uni.
Can you say what uni and what subject? That may make a difference. Some PPs say "the uni think he will cope" but really they may just want people on seats there. I don't imagine they know him well enough based on his application to know if he will cope or not. Apols if that sounds harsh.
Good post friom @WombatChocolate yes I agree with that advice.
And @rhabarbarmarmelade I agree re non RG unis as well. Both my uni DC are/were at non RG and both are having or have had a very good experience. This is Lboro and Leicester – but see also Lancaster, Bath, Keele, Kent – I know students at all these and don't see that a very much more amazing offer is availabel at Liverpool or Cardiff or Southampton tbh.