Well maybe you can safe him. Try to catch him and place him on wet paper. Check if the hump is "soft".
Then try to press the puss out of the open wound and put him in a small tank with salted water (as said in the other topic). He should not be stressed then. If it is an abscess you may have a chance to save him that way.
Eeewl that looks nasty. Its not in its intestinal area is why is able to keep on trucking. Any bulge like protrusion on the skin could be a tumor. Maybe catch it out and squeeze him maybe it full of pus. You could isolate him in a 10 gallon and treat with a antibiotic like penicillin or tetracycline might help it.
Go to walmart get some insulin syringes just ask the pharmacy counter for them. Take him out and stick the hump and pull back just a little bit on the syringe If you get puss treat it other wise if you just get blood or nothing leave him bee could just be a simple tumor and only be cosmetic.