I have seen this in my clients tanks when I get new accounts on synodontis petricolas, and others of the same family. You are right they are ulcers and tumors. I've heard that API stuff works I've also have seen good results on them with mela/Pima fix combo. It's easier on them since they are scale-less fish and are more sensitive to medications than most other fish. It's usually brought on by bacterial infections sometimes a type of fungus that attacks their skin under their armor also. Keep water quality up to par as dirty tank conditions can cause this( not saying that you don't just a suggestion for others to see) and a wound that got infected could have brought this on also make sure they are getting enough high quality food so they have the essential vitamins to help fight of the disease in question. Hope lil guy recovers quickly