Last year as I recollect Westminster gave boys an online maths and English paper, and then a fairly structured interview (my DS could read the questions she worked through upside down). SPS give them a 10 or 15 minute question for each of maths and English and then discuss those, plus topics drawn from the form your son is sent to fill in. This was a very wide and imaginative interview and I would have found it daunting but I think they pitch it for 11 yo boys well. And yes an object to discuss at Westminster too.
So apart from the object, nothing you can prepare for.
Just make sure your DS knows how to enter a room, make eye contact and shake hands. Then encourage him to think about what he would like to talk about - what is his favourite subject, what book has he recently enjoyed etc. Best to encourage him to think of it as a one to one talk with someone who wants to learn all about him, not an interview. Good luck!