He's just shedding. Horses always change colors durinh shedding season. I still see regular buckskin.
We have a buckskin at the barn who loses his sooty in winter and it comes back in summer, make his neck laced with black all over. It doesn't appear your horse is dark enough to have sooty though.
My friend's silver buckskin is shedding out and patchy all over with darker coat coming in under the lighter shedding coat.
Posted via Mobile Device