My son's completed grade 5 piano, and throughout all his grades he has failed two aurals. It seems that you can fail on any part of any grade but still pass.
Dd2 did flute grade 1 a few weeks ago, she hardly practiced, had group lessons, and really was not very good. Her last practices with me were terrible. Mistakes in every piece. She only had one aural lesson. Waiting for the results.
But my son says "everyone passes grade 1". I hope he's right. If your ds plays beautifully, it won't matter about the aural; the points he gets for his pieces will make up for any he may lose on his aural.
I bet he's fine. Hope it goes well.