Welcome Emma303 With DS1, interview at W was extremely slick. It's a very well oiled machine. Firstly, they will do their best to find an ex-pupil from your DS's school (irrespective of whether that is state or prep) to pick you and DS from reception and walk you through to wherever the interview is being held.
In terms of the interview, DS goes in first and will generally be asked a range of questions... interests, hobbies, opinions etc. He may be asked an/some academic questions but that in itself doesn't necessarily convey anything about how well he may have done.
Then it will be your turn to go in. To be clear, they are NOT interested in what you wear! So jeans or whatever you feel comfortable in is fine. Also, there is no comment/penalty for only one parent being there.
The interviewer will tell you how your DS has done. Sometimes this is specific ("he was great in Maths and got 90%") or more generic. If he is being considered for an academic scholarship, they often will tell you at this juncture (they didn't for DS1 and he still received a scholarship).
W in particular do not like being a back up, so choose your words very carefully. They appear to be less fussed about being In competition with T but are very wary of being back ups to those who think that their DCs will get into the Sutton grammars.
All in all, the process lasts no more than 30 mins and remember, they only interview people they are interested in offering too, so you should be very proud of your DS for getting this far!