Got ya. If the injury is healing that's good. My guess without actually seeing what's going on is something is spooking him. You say he panics when you turn the light on? Is that how he got the injury on his side? Sometimes a bare bottom tank will bother these social and aggressive cichlids. You don't want to take off the light so try doing a small amount of gravel scattered on the floor of the tank.
Get a bigger aquarium as soon as you can.