I was a Choral Scholar at Cambridge. I can't speak for all choirs, but I don't think being a "choral singer" rather than a "soloist" should necessarily be a hindrance - I'd expect the choral directors to be looking for voices that will blend (IME those with very "soloist" voices annoyed the crap out of me, always sticking out like a sore thumb - not appropriate for a lot of repertoire IMVHO).
However his audition would require him to sing one solo piece, and do sight reading and aural tests, so I think singing lessons would be beneficial to prepare in that respect.
Choral courses are not necessary - I never did Eton, I did join NYC (briefly) but I think that was after I'd been selected for a Choral Award (and only did one resi course, as my chapel choir commitments meant I couldn't attend any after that). I hadn't even sung in an anglican church choir before my audition (my take on how to sing an anglican chant psalm was met with rather a lot of amusement by the Director of Music auditioning me
), so your DS is already ahead in that respect. A lot of boys auditioning will have been choristers at cathedrals and schools, but not all - I suppose that was less the case amongst the girls when I was there, because those opportunities didn't exist so much.
And choose the choirs carefully - with a degree like Maths/NatSci (and CompSci too I suppose, but I didn't know any personally so I'm not sure what their workload is like) he really wouldn't be wise to join a heavy-committment choir. I read Engineering and was in a choir which did 3 services plus an additional practice per week, and it was bloody tough. I don't regret doing it for a minute, but I certainly put myself through a hell of a lot of pain doing so (and my academic results suffered).
There is a Choral Award open day at Cambridge on 25th April - I really recommend going along.
Best of luck to him! 