Just in case he's under some illusion: there's not really a lot of extra work to be put in to get into Oxbridge. For science/maths they'd expect at least 3 A grades in science/maths A levels and that's it really. It obviously helps to show you're interested by doing other stuff connected with the subject, but there aren't usually any extra tests to be taken. If you're son will probably get 4 A grades anyway then he might want to think about looking into Oxbridge (Cambridge obviously...)
The one exception would be maths which usually requires a STEP paper.
There really is:
a 2-5 day interview process which you need to prepare for thoroughly
an entrance paper set my the college
to get to the interview stage you pretty damned good As level grades (270+ in everything)