He looks great! I don't know if you'd want to do this on his face, but often times people with white-legged horses clip the hair close.
You could try cornstarch, I guess people do that. Really any white chalk would work. There's also White 'N Brite, which has purple dye in it that brightens whites. Its VERY purple, so its scary to see as you shampoo, but when it dries, it reflects the light, making your horse blindingly white. Be careful on his face, though! SILVERADO FACE GLO 8 OZ-Big Dee's Tack & Vet Supply
This stuff is the BEST! You can often find it at supply stores rather than ordering online.
Get clear, though. It works for both white areas and any color. My horse is a solid chestnut, and clear is fine.