I am not sure if the following has been said before but we found it useful to look at each Uni's actual admissions statistics as well as their entry requirements. Also, some Uni's have a separate admissions policy document which can give more detailed information than that given in the admission criteria given in the prospectuses. (IYSWIM)
The Oxford Uni Admissions Stats Page suggests that your son would be in a good position with his excellent GCSE results. Although! it shows that two thirds of those with 100% A* GCSE still don't get offered places.
I could not find the statistics for Cambridge, they are much more cagey with their information.
My DS is studying medicine at Sheffield and is enjoying it tremendously. He wanted somewhere with early clinical experience and wasn't keen on the shorter terms at Oxford and Cambridge. He got two offers after interview, one rejection after interview and he withdrew his application from Bristol as he already had his first .choice and they were taking too long. He had a high UKCAT which compensated for him having absolutely no GCSE's (or anything similar) as we had previously lived overseas.
I am sure the teaching at Oxbridge would be amazing but, personally, wouldn't be concerned about which Uni produced the best DR's. They are all good. . He needs to work out where he would be happiest, where would suit his learning style and where he has the best chance of getting in.
One thing we did,that we thought worked out well, was for my DS to go to the open days on his own. (or with friends) He thinks it helped him make up his own mind about where he wanted to go. Obviously he wanted our opinions on where would be a nice place to study but he wasn't overly influenced by what we thought. I realise your DS will have already attending any open days but I thought I would mention it in case other potential Medic Application DM's were reading?
I don't know about Cambrigde but I love Oxford and think it would be a great place to go to Uni. It's got a lovely studenty feel to the town and everything is nice and central. My son did a Week work experience at The John Radcliffe and was very, very impressed.
I have said it before, but I think there should be a degree course in How To Apply To Medical School. It's so complicated, everything changes year on year and not all the information you get is correct.
Good luck to your DS.