I'm worried, though. You're wise to pay attention. But your boyfriend may be right about waiting until it happens again. If he's in his 20s or 30s, the cause could be a number of things - none of them likely to be terribly serious. Red meat and processed meats can put blood in your stool.
Hemorrhoids can, and so can ulcers.
An infection called diverticulitis in the colon could also be the cause. In younger people, the most common reason for blood in the stool is a hemorrhoid or fissure very near the anus. In general, bright red blood is coming from a site close to the anus. Blood from higher up in the digestive tract, such as from a bleeding ulcer, will look black and tarry. Doctors use a simple color test to reveal occult blood in a tiny amount of stool obtained from an anal examination, and this should be done as part of every physical exam.