It definitely appears to be an allergy...especially if it just started this spring; there are alot of allergins this time of year (and fall), and one never knows when a horse will acquire an allergy.
What I would do, is have the vet do a skin scraping on it, and make sure that all it is is an allergy; rule out fungus' and other infection related things. Then treat accordingly.
I would probably also put something like Aloe Heal, or even Hydrocortisone on it for the time being, as either will keep the places moist, and help reduce itching and swelling (due to irritation)...maybe even wash those areas with a mild shampoo or soap, as that will help remove the allergins, and help cleanse the areas and keep them from getting infection.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."