Add salt to the aquarium.
Is it possible that he tried to jump out and it hit something?
https://www.monsterfishkeepers.com/forums/threads/arowana-physical-damage.691896/
") Improving healing rates:
The most important thing is preventing infection of the problem area or wound in bad cases. Although it is not very common for such to occur, keeping the water clean is important. High amounts of salt is not necessary. Use it moderately at the right dosages, e.g. 0.5%.
I have heard this many times, but have yet to prove it. Some owners reported significantly improved healing speeds when the temperature of the water in the tank is increased (30 DegC). It may be due to an increased metabolism that the body of the fish repairs itself more quickly and also tends to eat more."