Are there any white hairs sprinkled throughout his coat? Or on his face?
I have a bay-going-gray foal that it is really hard to tell in pictures he was going gray (at least until his face got more white on it) because those individual white hairs don't show up in pictures. (Your leg pictures are great, by the way, those do look like white hairs).
What about when you part his tail, near the tail bone. Any white hairs in his tail?
Those where the first really evident places on my foal- stray sprinkled white hairs, especially on the face (but no goggles oddly enough) and white coming in at the end of his tail bone.