I saw the close up of your pretty on FB and i am pretty sure he is just stained not a silver pied, i have 30 big whites here most all are stained somewhere it is that time of year. If you look at his crest it also is stained the same color as the back
Congratulations
He could just be stained, i have some with brown stains on their back and some with gray but they are molting so these stains on my birds will be gone soon, you will know for sure in a few months weather it is colored this way or just a stain, either way he looks nice.