Further thoughts, OP -
For maths, Imperial and Warwick are essentially equivalent to Cambridge and Oxford. These four are known as the COWI group and are the top tier of British maths.
There is a real element of randomness in Oxbridge admissions. Most qualified applicants, particularly in competitive subjects like maths, do not get an offer. Imperial and Warwick attract a good number of these superb mathematicians.
C, I and W use STEP but I and W offer alternatives. As I implied above that could be important, because only those with a high degree of a certain kind of intelligence will find that STEP preparation comes naturally. For most highly able pupils, STEP outcomes are linked to the quality of the tuition they can access.
And to be denied entry at this level in the summer because you fell at the STEP hurdle, which is the fate of about 50% of C maths applicants in a typical year, is a bitter blow.
FWIW, DH enjoyed Y3 and Part III at C very much. He felt Y1 and Y2 were designed for the ‘Clever Clogs’, students who could quickly see the trick. Only when he could start choosing more theoretical modules did he rise to the top. But he turned down the offer of a funded PhD at C for one at a red brick and he’s had a wonderful career in America and the UK at institutions in the tier below COWI.
He said recently he might have done better to choose W or I for UG, and then apply for Part III at C.
This is a long way of saying by all means your DS should apply to Oxbridge if he is interested. He should give it his best try. But it isn’t worth any angst.