I agree with another person’s suggestion to do your best to keep your horse out of the sun. Turn out in the morning before the sun gets too high and back in for a while and then turnout later when the sun begins to lessen in intensity again. I guess if possible make sure he is able to have access to his stall or run in shed as well. You may try using a slinky that fits over his head snugly and may not allow him to rub it off. See if you can get a used one or borrow one before investing in brand new items and wasting money. Also there is Zinx Oxide which is very effective to protect his sensitive skin from the sun. Only problem is that his face would in water occasionally to drink and I would ask the vet if it would be harmful if zinc ends up in his water when he drinks.