How is ulcerative proctitis diagnosed?

Concerning my husband, 64 yrs. Presenting problem:  Periodic bouts of painful bloody diahreha. Tests he had: colonoscopy.  The results were normal.  We are baffled. He gets attacks of 2-3 days duration of painful, bloody diahrea.   Then he stops eating, sypmtoms clear up for 2-3 weeks, then re-occur.  He wasn't given a diagnosis. He asked his drs., "Are my hemmeriods causing my rectal bleeding?"  Both his G.P. and his Gast. Dr. reply, "No. Def., not." to his question.  He is convinced that his hemmeroids are causing his (?) compelling, repititive bouts of painful, bloody diahrea. Why don't the drs. tell him he has ulcerative proctitis (which is my diagnosis, but, then again, I'm not a doctor.) Note:  He also has Parkinsons. His neurologist said Parkinsons causes constipation.