My Six Year Old Son Started To Bite His Nails

My Six year old son started to bite his nails since he started Kindergarten and six months after that he also started to rub his fingers to the point that he has calluses on one of his thumbs and a couple of his fingers. He has always had a tendency to pick at things, for example I always have him wear socks because if I don't he picks between two specific toes on his right foot. What do I need to do? I took him to a therapist and she concluded that these are all habits that he needs to grow out of, but to me they seem to be getting worse.