I'm almost definitely going to regret saying this, but if your ds has been interviewed post exam by more than one school, even if it is a rejection and not a WL, it is not the end of the process. There are still discussions to be had, and there will still be movement until the end of March and beyond. It will not be easy, it will not be fun, but the only people I know who didn't find a suitable place at one of their schools were those who gave up as soon as they got their letters. And that is also an understandable reaction - it has after all been a long haul since September. But you're able to see the last couple of years threads and you can see how late movements come through. And there'll be plenty of virtual support on here too.
The schools do not interview if there is not something that has been flagged up in terms of potential, but further investigation and discussion is needed to see where the gaps are. Some schools won't offer if they are obviously your insurance school for example. Others are dubious if you haven't done their open days and tours. It may be that your ds did better at one paper than another and more evidence is needed.
It is unfortunately a different ballgame if you haven't been interviewed. I have to say that IME to say that 2/3 interviews at W&T end in an offer seems a bit on the low side, but my view may be skewed by being in the private sector. I would expect at closer to 80% and probably more of those interviewed would get an offer.