Before you do anything else...Test the water for PH, Ammonia, Nitrite and Nitrate.
In 90% of cases it is the water that has led to deterioration of the fish THEN something like this happens due to weak fish.
Also was he in a group - (they should be kept in groups) it could have been simply bullying but I would put my money on water quality.
Do not forget that an Ammonia spike could have been and gone by now...did you clean out the tank or filters, add any recent fish or change anything withn last week or two?