My pleco used to get that all the time. It kind of came and went day to day. Does he have a nice spot out of the light where he can go? If not try to make him a little hut out of something so he can be in the dark when he wants to.
Is the white area slightly raised? Did it start on the dorsal? If so, I treated this like columnaris, bacterial infection, and super ich, and all 3 eventually failed. Out of my original 12, I culled all but one. That one is thriving at the lfs. I spent hundreds of dollars and 4 months trying to treat. I even upped my tank to 90 degrees and added salt and copper as a last ditch attempt. When it failed, anyone who didnt look perfect was culled to prevent further infection.
I'm not trying to scare you, just letting you know what some have been through. Cories are tough to diagnose, as a lot of disease presents almost exactly the same with them. Some have had great success, others have lost the battle. Good luck!