Your guy could have the start of pop-eye. Is the eye sticking out at all?
Usually physical damage (maybe the sailfin rough him up a bit) or poor water qaulity is to blame.
If its pop-eye, good water quality and a good diet should clear it up.
Watch for secondary infections. if you see some sort of secondary infection you could treat him with a gram negatvie antibotic like maracyn 2.
But, good water quality will help improve the condition