Thanks for all your comments. It's always helpful to hear others thoughts.
I decided to check when exact dates of the entrance exams are and it turns out that the school he really wants to get into has been brought forward their exam until mid November. The 2 local Indies are the first week of January. This does make the G5 do-able in December. But we do have the dilemma of him getting very bored with playing the G5 pieces for so long. I do kind of wonder whether Teach is kind of testing to see if he'll be ready in July by how quickly he can play to a fair standard (which has happened before).
Also checked the ABRSM C session exams and it turns out that we could actually be assigned one the week before our Trust exam week. So it wouldn't really be worth going through the hassle of traveling to a strange exam centre, different accompanist etc for the sake of a few days!
DS, DH and I have agreed that we will see how well his practices go before next week's lesson. Night before the lesson, I will contact Teach, tell him our worries/dilemma and see what he says. If he thinks G5 won't be possible, then we'd want DS to be playing things other than the G5 pieces. He may agree to do an extra private lesson in half term, and decide then as the deadline is first Monday back at school.
I think you know that DS is highly motivated by the exam process and does want to get all the certificates. He's also getting very used to the high marks, so, as you say TDM, it could be a good lesson if he didn't do so well, and he realises that the world doesn't stop.
Now we have the worry that he has about 7 weeks less to prepare for his school exam! And I'd got it all planned that he could do an Indie exam as preparation for the Tiffin one!!!