Sounds like he could be sick just like you think. If you can, setup a sick tank and begin fresh water and medication (such as paraguard). If you dont have a separate tank then you can do it all in the same tank. It wont hurt your other fish and if they are sick too, itll help them as well.
40Gal Planted- 10Gal RCS Planted
6 Julii corys, 1m Furcata Rainbows, 2f 1m Bose Rainbow, 2 mystery snail