Use baby water proof sunblock! They also make sunblock sticks to use. I had a paint with a pink nose and everyday he got sunblock on. We never had a "burnt" peeling nose again! He was very greatful and soon learned that sunblock was his friend. And its friendly to their skin.
They do make those flymasks like buttermilk said. Its good extra precaution with the sunblock! I would use both incase your horse is frisky and likes to "loose" his masks! Also the sunblock is good when you are riding.
To help heal up his nose from the sun burn and peeling, use an aveeno lotion its water proof, fast healing, and great for sensitive skin as we all know horses have.
Good luck with your poor horseys nose, the one downfall to pink noses :)