Do a big water change, add salt (5-6 tablespoons/10 gallons). See how he does in a few days. If no improvement, then post some pictures up so we can get a better idea of what it is.
BUT, once he is doing better you will need to get more discus since they are schooling fish. 1 will not do very well by itself. I recommend no less than 4. And yes, a bigger tank is most definetly necessary if you are wanting to stick with discus.
-Ryan