Looks like a bacterial infection...and a pretty bad one...
How long has he (Panaque maccus?) had it?
You can try methylene blue or make baths with Nifurpirinol if you have.
Also try a search on infections.
Melafix is another medication that I have seen work miracles, but it seems you are set with the medication area. Frequent water changes are a must though. Just keep that in mind during the treatment.
Just to let you know ... I had a real bad experience with melafix and my common pleco. He was allergic to it! So watch him carefully if your going to use the stuff, but I agree it worked wonders on my koi's. Good luck
It's head (upper body) swelled up to the point that his eyes were sucked into it's head. He also moved in eratic, jerky movements. It took me a good week in a emergency hospital tank with great water conditions to get him looking and acting normal again. I never expected this reaction since melafix is natural. I had posted in this forum to see if anyone ahd ever experienced this and didn't really get a reply.