That looks like a bacterial infection. Is he the only one in the tank or is he with another tank mate? If so it could be due to aggression. Keep the water as clean as possible, this will help the infection. Ammonia and Nitrites should be at 0ppm, very important now. Follow the advice with the salt and temp. The primafix should also help. I really hope he makes it, my fingers are crossed. Keep us updated.