There are many options, first off it would be nice if you could provide a picture of the tank. The color of the jag will give away stress as well we could see the amount of hiding spaces in your tank.
Keep the tank lighting off provide hiding place and try sinking pellets and other food he can eat from the substrate.
Another thing that might help is another fish depending on the size of your tank. If another Jag is hard to get, try an Oscar fish respond will to competition for food.