HI, WELCOME TO FISHLORE!
Most plecos need huge amounts of ruf***e, algae, wood, vegetables of all sorts. What has yours been eating? Has his diet changed in the last two weeks?
I would suggest not feeding him for 4 or 5 days, he may not be eating anyway, but just in case. Do you have driftwood in with him? If you don't get some. Make sure it is safe, anything you get from your LFS should do. When you start feeding again, give squash slices, algae waffers, or slices of cucumber. You might try a round of parasite medication as well. Hope he gets to feeling better.