My comments are based on advice that I read on their website. It isn't there at the moment, it was in the section where you submit the application, and applications are not live at the moment, but I just looked through my emails and found the document, as I thought we might want to refer to it again.
The blurb says: 'We receive a high demand for places and cannot accommodate every request we receive. Places for work experience will be distributed between local schools with whom we have partnerships, candidates known to GOSH staff and applicants contacting us directly.'
So I'd have placed your son in the 'candidates known to GOSH staff' which I assumed would have a better chance. I found that section quite odd as you'd think they'd want to prioritise those without the connections, which is why I wondered if they meant patients rather than those who know the staff through personal relationships, but the only person I have ever come across who was successful is someone who mentioned it in a TSR thread - another med student was asking about electives there and they replied saying the'd done WEX there and recommended it, so it seemed genuine. I asked if they had any tips for success and they just said they knew a consultant there, and that was how they got in - it was back in 2017 though.
I'm glad your son got some WEX though - DD has a week lined up for Feb half term too, in our local hospital, but in their respiratory medicine team, as their trust doesn't allow under 18s in A&E or maternity or paeds. Her two preferences are A&E and paeds! Though after a story I read on TSR of a 16 yr old volunteering at a hospital in A&E, I think it's quite a good thing DD won't be in there. I took her along to her younger sibling's outpatient respiratory appt at St George's in the summer, and explained to the doctor we saw why she was there. The doctor then took quite a bit of time explaining everything she was doing and it was great - I learnt a lot more myself as a result. I'm trying to get her to come along to my smear test too - head end only! But she was less keen on that one! She's also got 2 days of virtual live WEX with Southampton University Hospital tomorrow and Wednesday, so maybe she's being a bit greedy chasing the GOSH one too. She's decided against putting her name in the hat for Head Girl they have to put themselbves forward for it in DD's school) - says it is a big time commitment and she wants to focus on getting the best grades instead.
Which med schools has your son applied to?