pannetone - I'm sorry I didn't return yesterday. Personal circs. intervened.
Your son should look into the Cambridge Special Access Scheme as a possible application route.
The people interviewing in Oxbridge are (mostly
) human, and want to get the best students they can and to conduct the best interview they can. If your son is anxious that the interview system would hinder him, and you believe that this would be a direct consequence of his SN, then alerting the interviewer to that - his referee will fill in a special form - is the best way to address this problem in the first instance, I should think. At the very least, a phonecall to the Admissions office would clear up any questions you might have.
Anecdotally, while at Cambridge, I interviewed a candidate with AS (mentioned in ref., but not - interestingly - through the CSAS). It was immediately clear that s/he did not find the interview room a pleasant environment (though few do!), but I was also anxious to make sure s/he was able to show her/himself in the best light, so I asked her/him what s/he wanted to discuss about her/his studies, and we progressed from there. S/he was obviously very clever, and quickly found ways of showing me this. We worked it out together. A flexible and humane interviewer (who is sufficiently clued up about the candidate) will do their best to make the interview a success. Long story short: s/he got the offer; got results; came up to Cambridge; got a 1st; did an MPhil... So... as I said, your son should go for it! (Apologies for "s/he"-ing everything - seems more anonymous that way).
Just out of interest, what subject does he want to study?
The interviewers are usually as keen as the candidates for the interview to go well. We spend a lot of time interviewing. It is very tiring and administratively quite burdensome. At my current institution, we interview from Nov/Dec through to March, and in that time, I will have processed hundreds of UCAS forms, and personally interviewed at least 50 people (most members of the dept. also interview). Therefore, IMO, it is much nicer to have a good interview with someone than a bad one! No-one likes awkward silences, or tears
, etc.
It is true that sometimes a student will freeze up so badly that you cannot get proper impression from them of their capabilities. If you are interviewing - as at Oxbridge - for a supervision system, it is possible that the interviewer will conclude that the candidate is not up to its peculiar pressures. However, in the case of your son, were his referee to make clear that in situations of particular pressure, rather than at all times and as a rule, your son underperforms, then they would work to find ways to get round this.