Is it a fungal infection? Sometimes, that can be the cause. Other times, nails dystrophy for no known reason, due to an abnormal blood supply to the growing root of the nail. If there is no other symptom like pain, difficulty in walking etc, you can wait before worrying so much. You have already been to a dermatologist. You might want to go back to the same doctor and ask him to reassess your son. Changing the doctor won t serve any purpose unless you know of a definitely better doctor.