Sun burn would be my first thought too and depending on your horse's overall health and what he may be eating out in the pasture it's a definite possibilty. Certain plants make horses more sensisitive to sun exposure and white areas tend to be more sensitive anyway. I would try keeping a heavy sunscreen on his nose or keeping him in during the day and turning him out at night to see if it helps.
A second opinion from another vet would also be a good idea since this has been ongoing for so long and your vet hasn't offered any new options.
Licensed Veterinary Technician