There is not much magic to it.
For the moment the main thing he needs to do is work hard for his GCSEs.
Second, check he chooses appropriate A levels next year that will not exclude him from the subject he wants to do.
There is a wealth of information on both the Oxford and Cambridge websites, on the faculty pages, the college pages and so on. At some point in Y12 he will need to choose which one he wants to apply to (you can only apply to one or the other) and which college and subject. There will be open days and so on. For certain subjects you can apply to spend eg taster weekends there.
Remember that what they are really looking for is not hundreds of extra curricula activities, D of E awards, loads of school captain type positions, blah de blah. What they want is someone who loves their subject and will be interesting and fun to teach.
If he knows what he wants to do he should start taking it further and reading around the subject.
Try to tell him not to set his whole self esteem by the question of whether he gets in or not. At the end of the day there are lots of good candidates and there are just not enough places for them all. If he doesn't get in, but is still keen, he can still work hard for A levels and then apply again, or to the other one, post A level. I know lots of people who did that. In terms of maximising chances, I don't think there are any magic tricks. The interviewers are teachers and people, they choose those with the right academics, the interest in the subject and who they could imagine enjoying supervisions with.