if you can, shift him to a hospital tank; means you can treat a smaller body of water, as well as avoid infecting your other fish. Even if its a 20 gallon for him that's fine, since ideally he wouldn't be in it for too long.
If you do choose meds remember to remove any carbon from your system. There is a sticky thread in the stingray forum with a list of Ray safe medications. Avoid anything with copper