I'm noticing more and more now though that he has some random white hairs... everywhere. All over his neck, back, shoulders and hindquarters. Very faint, hard to see. I'm attaching a very recent picture (not the best but I will take more) from a photo session I had with an amateur photographer friend of mine... so pardon the editing :P
He's only 13, by the way, so I don't think that it's age? And wouldn't that type of grey hair pattern appear more on his extremities?
Regardless, I hope someone has an idea!