First, he is not a Shubunkin, he’s a fancy calico goldfish. Shubunkin’s are single tailed fish. It appears that he has gotten stuck under something and rubbed himself raw. I’ve had one do that before. Do daily water changes of 70% for the next week and see how it looks at that time. I would recommend using some Melafix according to bottle also. I rarely suggest Melafix but it is a good astringent to clean the wound and has some anti-bacterial protection too. It normally works well on goldies. If you see the wound worsening then you will need an antibiotic such as Kanaplex. I think he’ll be fine with the clean water and Melafix. Note: Do not use aquarium salt in a wound like that as it will burn the fish. Good luck!