Thats 3 weeks old? is it getting worse or evening out? It looks pretty bad from what I can tell, but to effectively treat you must remove him to a hospital tank. The treatment would be with an antibiotic like kanaplex or one that you use. The hospital tank would have no porous material like rocks and substrate and must have a cycled viable biological filter. If you can not provide him with an adaquate hospital tank, moving him would do more harm than good. Antibiotics never work dosed in display tanks. While in this case I would suggest treatment with an antibiotic, you must do what you can.. Eels are very resilient, I have seen some recover from such wounds on their own. The key to recovery on their own is 1st off, water conditions must be pretty much perfect. A fresh healthy diet also helps, I suggest human grade fresh seafood; shrimp, salmon, squid, mussels, clams. You could also stuff food with Saki-Hikari marine carnivore pellets which has a high amount of vitamin c, which is good for the immune system.
