He had ulcerative colitis. People more or less use "having an ulcer" as slang for being in a bad mood. Maybe he was like the giant with a thorn in its foot - so

He had ulcerative colitis. People more or less use "having an ulcer" as slang for being in a bad mood. Maybe he was like the giant with a thorn in its foot - something bothered him, and that made him less than in a good mood.