MachineBee, I love the sound of your youth immersed in music. It's so nice to hear the real passion and love towards music coming from the child. 
As I understand OP is after grade only for securing a scholarship, I just thought to share my two penny...
I heard some schools request actual exam pass but as long as I know most schools are happy without it. DS applied for a music scholarship (audition/exam just done this year) with no exam experience for his first study. I mean ZERO.. We had to submit teachers' references so his playing standard and commitments were mentioned there. He was selected for an audition, and won the major scholarship.
For your information, we live in SW London so I know how fierce the competition for a scholarship at independent schools is.
I was not sure how much was true of what they said on their website (they stated exam pass was unnecessary) but thought if DS didn't get invited for an audition because of his lack of exam experience, then the school was not right for DS. We wanted DS to go to a school with real celebration of loving music than a certificate obsessed school.
Do you have any particular school in mind? If so, why don't you call and ask them the criteria (I did)? If they say actual exam pass is not required, your DS can focus on developing and mastering and consolidating his skills to rise his actual playing standard than wasting his precious time chasing g5 unnecessarily. He can take higher grades after secondary school is sorted, and probably gets distinction at ease.