My daughter liked "Paul Harris 'Improve Your Aural' Books", too. I also got her books called "Aural Test Survival Book" from Grade 4 onwards. She enjoyed reading the Survival guide G5,too.
I think the key to prepare a child for aural tests is .... easier said than done... do not leave it till late, and have some sort of aural training regularly. One of my favourite aural exercises is to play a 3 note chord, and sing the lowest, middle and highest note.

If you have a piano, this is highly recommended - you can play the piano and challenge your child, and s/he can play a chord and ask you.

You can extend this to naming intervals, too. I have also found that Trinity's Aural book I (covers G1-G5), which comes with a CD. Although Trinity's syllubus is different, the book & CD give you good aural training that you can do with your son.
Wishing your boy good luck with his G5!