I notice that boys that didn't achieve the pass mark are sometimes successful at appeal because they have a brother at the school already.
I know of one boy who is currently in year 7 who was offered a place in those circumstances. Trafford resident, non Catholic, didn't pass but has a brother at the school. I'm presuming this place was offered after appeal but I don't know what formed the appeal but I would imagine having a brother already at the school would have been part of that appeal.
Sorry, not very helpful I know and different cirumstances to yours but I didn't want to just read and not comment. I wish you and your DS good luck if you do appeal.