What Causes Blood Clots In Urine?

My son is a healthy 26 year old male who recently started peeing blood. Unbeknownst to me it has happened about 3 times in the last 6 moths - usually after drinking during a party. He is not a big drinker. This past Friday morning he was urinating not only blood but clots as well. He went to an urgent care and they tested for UTI - which came back negative just now. He sees his primary doctor tomorrow. What are the possible causes of this in such a young person? What troubles me is both my mother and father died from cancer - but both were heavy smokers. My son has never smoked.