Hey, Welcome to fishlore.. hopefully your pleco will be doing good soon. just have a few questions about the tank.
How big is the tank? What other fish are in there? How often do you change the water and how much do you change when you do? What water conditioner do you use on your tap water? Do you have a liquid testing kit? What is the
ammonia at now?
What did you use for stress relief - Stress Coat?
What I would do is start changing 25-35% of the water every day and treat the water with a conditioner called "Prime". This should help bring the ammonia down and help your Pleco be safer from the toxins in the water. The time to test the water would be right before a
water change. If you want to order a product called "Fish Protector" (petmountain.com) and/or another one called "Vitachem" (petmountain has small bottles, drsfostersmith.com has big bottles), either or both would help him recover faster.