It sounds like your guy most likely has a fluke problem. Further back I had mentioned that clamping one fin to the side IME has been the very first sign of flukes. That gets followed by using one gill. While always present in a tank, Flukes can attack a fish when its been stressed. Without a scope it can't be diagnosed exactly, but the symptoms say flukes to me. If none of your others are exhibiting the same one gill usage, I'd remove him -- your 10 gal. may be fine -- but you will have to closely monitor your water and do frequent WCs if he is larger.
Do you have Hikari Prazi Pro on hand? I'd start with that, its specifically for gill flukes and also treats certain types of worms. Its also very gentle on fish and does not impact your bio filter. Note that Prazi Pro is sometimes ineffective these days though, as flukes are becoming resistant to it. But first time usage, it should do the job, as long as treatment is for a long enough period of time to kill all emerging hatches from eggs. Another treatment for flukes is pure Praziquantel powder, more potent that Prazi Pro, but also a PITA to mix. It can be purchased at
www.jehmco.com for a decent price, and if I remember correctly, many people mix it with Vodka.

But the Discus don't get drunk! I've never used it myself.
If you don't have Prazi, you can try Quick Cure, which can eradicate flukes if the infestation is light. If you use QC, make sure to increase aeration in the tank.
And, as always, monitor your water quality. Do you have Prime or its equivalent on hand? You may need it to detoxify ammonia/nitrite in the QT.