I don't think there's any luck involved in getting places at the Brit. It will come down to talent and showing the right aptitude and mindset for the courses.
I'm intrigued that you think MT means likely employment - it's really extremely precarious. If you consider how many hundreds of new degree graduates there are every year in the UK and how few MT roles there are available. There's cruise ships, and teaching as well as actual theatre roles but very, very few make a living from performing.
Did your son try out for any West End roles as a child? There are literally hundreds who audition - not as bad for boys as it is for girls, but still extremely competitive. A lot of the successful WE kids will be looking at Brit for KS4 or KS5.
On the music side, I know a number of children who are looking at Brit for KS4 and in particular KS5. Most play several instruments and sing to Grade 6-8 standard. They are already writing and doing live performance opportunities on a regular basis and have been focused on this. Songwriting can be a much more steady career - IF you are lucky enough to have the ability to write songs that are commercial and sell, and the right contacts to be able to market yourself successfully. Looking at the kids I know who do this, it's not something they choose to do, it's something they have to do if that makes sense. I would say that KS5 is likely to be extremely competitive for Music as a lot of musicians will be more than happy with what they can access at normal schools and extra-curricular before 6th Form.
In terms of what they want for the Music strand: an interest in music theory, composition, music tech and a high standard of performance in voice and/or an instrument.
If he's not sure, why not wait under KS5 to apply and be sure of going for the right one?