My 12 year old son gets poison ivy every summer and about a year ...

My 12 year old son gets poison ivy every summer and about a year ago we took him in for a shot. The shot left a dimple in the upper part of his buttocks. Not a big deal at the time. Now almost a year later this has grown to the size of a half dollar and skin looks pinkish in the center. Does anyone know how they treat this, if at all? Poor kid, you know he won't be wanting to undress in the locker room. We need to do something about this. I also was not warned of any side effects let alone a disfiguring problem.