He is just stunning!!! Is he a Belgian cross by chance? Along with the sabino coloring, it looks like he may have the Pangare modifier that lightens his lower body and his muzzle. Dapples are usually caused by nutritional factors. It is usually fat or very healthy horses who display dapples. Pangare:
Pangare is a modifier that acts by lightening certain portions of a horse's coat, usually the muzzle, the belly, the inner forearms and thighs, and sometimes even the chest or around the eyes. When it acts on a chestnut coat, the horse usually will have a flaxen mane and tail. It can act on any color, and varies in intensity. It is often found in pony and draft breeds, such as Haflingers, Fjords, Exmoors, and Belgians.
Pangare modifier on a chesnut/sorrel horse
The modifier on a bay horse
You can find all kinds of good sites if you google horse color genetics.
Here is a site that I really like, it has tons of colors and patterns that I had never heard of before. Equine Color Genetics