2plus2 from what I've read it seems they use the MAT results more than anything else to decide who to invite for maths interview. So if you get above a certain score you are interviewed, if you are a little below it and your grades are very good or you have some mitigating circumstances then they may invite you to see if you were having a bad day when you sat the Mat, but in general, to get an interview it's essential to do well on the MAT.
After that, it's difficult to know exactly what goes on. They seem to score your interviews and amalgamate that score with your MAT score and also your prior achievements/UCAS reference, but it seems there isn't complete uniformity across colleges as to how much weighting is given to each. St Johns openly say on their website that the MAT score and prior attainment/reference is most important and interviews carry relatively little weight, whereas an old Worcester maths tutor's blog I found seemed to imply that there the MAT is largely disregarded (except as a selection tool for deciding who to invite to interview) and that at the interview stage everyone has a blank slate - though they also give an extra test to their candidates at interview too. (Do note that this was from 2007 so their policy may have changed). Everyone who gets an interview will be interviewed by at least two colleges in any case, so perhaps slight differences in how they prioritise weightings doesn't matter too much. I presume your ds has already chosen a college?
I've just googled and found the blog again, it's here and may interest you, as he seems to have a particular interest in finding mathematical talent in state school pupils that may reassure you.
So for advice: know the MAT syllabus really well (I think it's largely C1 and C2) and do all the past papers. Ds found there were enough on the website to practise on, but on TSR people do complain they run out sometimes - this is a bit of a problem since they're not very similar to anything else. The interviews will be mathematical too, so just do lots of maths and be prepared to solve (or attempt to solve) less straightforward questions than regular A level ones. Although the questions are likely to be trickier they are unlikely to be on topics not yet studied, or at least not on things that can't be figured out if you keep a cool head and apply the knowledge you know.
I can't think of any other advice at all - they're looking for good mathematicians, so you need to be good at maths and that's all really. Good luck to your ds.