Long nose fly mask with ears that has UV protection. The higher the UV protection, the better.
If your horse has a lot of pink skin around the eyes, a Guardian mask will be better: Guardian Mask Product Information
If he won't keep the mask on, slather him with zinc oxide (diaper rash cream). It's sticky but it lasts longer than regular sunscreen. The generic Walmart stuff has about 40% zinc oxide and is cheaper than the Desitin Max strength. It has an SPF of about 60 by itself. It's also nice for rubbing on the sheath/udders and really keeps the bugs out of there.