How did he get this, aggressive tank mates or water quality problem? Did you test ammonia, nitrite, nitrate, pH?
Make sure you have good water quality, then I would use Microlift Artemiss to heal the wound/ulcer it is safe for catfish.
I've never had a fish with that kind of ulcer/injury but I think I might look to an antibiotic to stop it getting bigger. Lots of large frequent water changes are always my first step.
A couple things I see is large gravel that can hold all kinds of poo, detritus & bacteria (do you vacuum it once/week?) & he seems quite thin. That maybe from your pic's angle...
Tell us all about your tank, size? how long have you owned your tank & this plec? What size is your tank & fish? Water changes, how much & how often? Do you have a test kit? What does it say?
