He hasn't been graying out at all. This particular member has been saying the same things for 2 years with no proof...
I too am of the belief that Nimr is greying, but it is difficult to see for sure. His colour has changed significantly since you purchased him. You can see the changes in his shade and especially in the facial marking when you compare the photos.
Here is is before you purchased him. Note the clarity of his blaze, and the warm tones to his colour.
Then there are the more recent photos. You will note that his coat definitely has a far more flat, cool tone to it, and his blaze has definitely become far less distinct as he loses the colour in his face.