Horse Veterinary Questions? Ask an Equine Vet.
Do you have a picture of the swollen area? Is he neutered? Is it still swollen?
Has he had any trauma that would explain the swelling?
One of the things that comes to mind is the bulbous glandis that is normal anatomy for a male. This is a swelling on either side of the penis that is apparent when is gets excited but goes away and is not visible the rest of the time.