Turtles shed both their skin and their scutes (individual sections of shell) as they grow. So this is very normal.
Just eating fish is very unhealthy for him. Try getting him onto a reputable pellet every other day as his staple food, and veggies should be given daily. As owners, we sometimes have to use tough love in situations like this to get them eating what they should. Withholding the foods they like is sometimes the only way we can do this. I wouldn't give any more feeders until you get him on a healthy well-balanced diet. In the wild, RES can go several days without eating so don't worry about withholding his fish. He will eat the pellets and veggies when he's hungry enough.
