What do I need to sue urgent care ?

My son was taken to urgent care for his broken index finger. General physician did his split and was given an appointment for a week later as per her she talked with the hand specialist and they suggested that it will be ok but now when he was seen after a week they are telling us that usually a next day appointment is given for such cases and everything gets healed perfectly. Now his finger will be crooked for whole his life which could impact his sports career and emotionally is impacting him. I feel some one is lying in this case and its pure negligence on the part of urgent care negligence that he will have this deformity for life.