All pets can be over fed to death if they are fed incorrectly. This happens to much smarter pets like horses, dogs and cats, and it even happens to people.
When I had a pleco, I fed it blanched spinch and kale, although cucumbers and certain squash works as well. He ate all the time and grew a lot, but did not get a swollen belly. He would sometimes swipe pellets from the surface, but often he ignored them when he was not hungry. Keeping a regular supply of spinach in the tank kept him away from taking pellets. I'd usually place 10-15 large leaves, rubber banded to a piece of flat slate in the tank every 3-4 days.
How much to feed?
Plecos are grazers. Not in the sense that they eat grass, but in the sense that they are, in their natural habitat, eating more or less continuously. This means that they want to have at least some food all the time.
However, with plecos as with many other fish and humans, overfeeding is much more of a problem than underfeeding, and aside from that, food that is breaking down in the water will affect the water quality in a negative way. The goal is to give sufficient food, without overfeeding. The best advice is to start with a small portion. If it's all been eaten overnight, it's probably OK to feed a bit more next time.
https://www.planetcatfish.com/shanesworld/shanesworld.php?article_id=294
http://aquariumtidings.com/common-pleco/
What is he eating besides algae?
Does he have real wood (e.g., raw driftwood) in the tank?