It's a bit difficult to tell with the information you are providing, as we cannot accurately judge the seriousness of the injury. Please answer as many of these questions to the best of your knowledge so we can help further.
Do you have any pictures you could show us of the injury? Pictures would really help us judge the severity of the injury.
How deep is it? Is there any evisceration? About where on his side was he cut? On the side of his head? Near his gills? Near his tail? Did you see any blood in the water? Is he still bleeding? If so, how long has he been bleeding now? Is it a vertical or horizontal cut? Clean edges or ragged?
How is he acting in the tank? Is he still swimming vertically? Is he as active as he was before the cut? Is he still eating as much as he was before? Does he spend an unusual time floating at the top of the tank, or laying on the bottom?
What size tank are you keeping him in? What is the water temperature? Do you have a filter in the tank? Do you keep aquarium salt in your tank? Were/are you treating him with any medications at the time, and if so, what are they? Does he have any other illnesses currently?