Okay,
First, lets set up a hospital tank. Move him out of his current home to prevent infection to his tankmate. A 10 gallon will do fine, even better if you can turn it vertically tall. Use paper towel as substrate and change it daily. Add water dish, thermometer, hygrometer. Cover the back and 2 sides of the tank with a green or other background covering to reduce stress.
Can you add a good photo for us? Do you know if he got hurt on anything? Was it sudden or did it slowly get worse?
We will try and help.