I will assume it's a common pleco. I hope you have it in a large tank (55g or more), or have plans to move it to one eventually. Common plecos are fairly vegetarian, and also nocturnal, so it's common not to see them eating during daylight hours. If you have plenty of algae in the tank, you shouldn't worry about it starving, and may only want to give him other foods once a week or so. You can feed them sinking algae tablets, and occasional slices of cucumber or zucchini. Just remember to take the vegetables out the next morning or they can foul the tank. All plecos like to have driftwood to nibble on for fiber, and a cave or other sheltered area to hide in.