My DC did have a choral scholarship at Cambridge til recently at a college with a three service a week commitment. he got an awful lot out of it: it added structure to the week, it provided free formal halls, it paid for singing lessons with a wonderful teacher, it improved his sight reading beyond recognition, it gave a slightly different group of friends from his main (subject based) friends, and they went on an annual tour, normally abroad, which was a great bonus.
Of his co-choristers, some had grade 8 and diploma level singing, and some had no exams to their name, but often had been in choirs throughout their schooling. I don't know that having the exam would make a difference, as really it is about how the singer auditions and how well they will blend into the choir. However, music exams do show a certain standard is reached and are certainly not a bad thing.
The negative side is the application process : because you apply for the choral scholarship before you have a place (at Oxford), or pre-audition (at Cambridge) quite a lot of time is spent on this process and it can be very disappointing to get the promise of the scholarship, and then not to get a place at the University. The choral trials take a good deal of preparation, and a chunk of time out the beginning of the Autumn term.
So, if you think he is good enough to have a good chance of getting in to the University, then it is well worth applying. Work on his sight singing.
Think about sending your DS on an Eton Choral Course this summer - I cannot recommend it more highly : they are great fun, and give you a good feel for whether you might like the life of a choral scholar, and for what the audition process might be like. The teaching is fabulous and the social side is great.