I wouldn't call it an emergency at all. The vet might want to sedate and then remove the nail on top so that it grows back more normally. If the dog lets you, I would maybe trim the top part as short as I could for now and just keep him from licking it to see if it starts to heal on its own. Thats all the vet would do anyways...
ETA: If it is infected, then a vet trip would be warranted. They would probably give antibiotics for it regardless.