OK, here's some info from the fish experts at the aquarium shop I used to work at:

First thing you want to do is test the PH and O2 (nitrite) of the water. Very important. If you don't have an aquarium shop or a PetSmart near you, you should buy a kit and test and test the water at home. It may be as simple as doing a 1/4 to 1/2 water change every other day for a couple of weeks. PetSmart does the water samples for free, just bring in a few ounces of the fish tank water (don't test just after a water change, you want to test the pre-water change conditions of the water)

If the PH and O2 are at a normal level then you need to start the pleco on either MelaFix or Wound Ease and follow the instructions for dosage on the package. Those medications are made by the company "Aquarium Pharmasuticles". The Wound Ease helps to rebulid the fishes natural slime coat that was damaged from jumping out of the tank. The MelaFix helps to clear up bacterial infections, which is what it sounds like he has by your description. The MelaFix also has medicine in it to help rebuild the slime coat.

The pleco should be removed from the tank containing other fish because the other fish may be feeding off his bacterial infection, thinking it's fish food. That will eventually make the fish worse and worse until it dies. He has to be seperated until his wounds are healed and he has cleared up his bacterial infection. If you do put him in a quarantene tank, make sure you are real good about water changes, because "new tanks" are always difficult to keep the PH and O2 levels at a normal level.

Good luck. I hope the fishie is OK and makes it through this OK. Please update us on his progress