I suggest monitoring him closely. What is the temperature? If it is lower than 26oC or the climate is cold there with your tank having no heater, chances are it may have signs of ich. Rubbing on things are usually the first sign.
What I would do is vary the food menu and maintain clean water to strengthen all your fish's immune system so they won't succumb to whatever disease may come.
You may see fish rub against something at times. I guess they can have an itch, like us. But watch the fish closely, as Blue suggested. If the same fish, rubs frequently daily, then something is irritating it.
Rubbing is a sign of irritation for one reason or another. Before doing anything, especially medicating, please post your water params for ammonia, nitrite, nitrate, and pH. What size tank is it in? What other fish are in the tank? How many? How long has the tank been set up? What is your maintenance schedule?
The more info you post here the faster we can help you.
VERY IMPORTANT: do not medicate until you know what your water params are. If there is ANY ammonia or nitrite, or high nitrate (above 60), the meds can be toxic. Also, depending on what other fish are in the tank, not all meds are safe for all fish. If you post the info asked for then we can help you to sort it out.
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