Also possible it's Melanosis. Just go to www.endoatlas.com, select "Atlas Index," then select "Colon," then choose "Melanosis." The picture on the far right reminds me of your father's situation. These white spots CAN be caused by overuse of some laxatives, and will go away if he stops taking those. But the gastro guy will probably consult with someone and figure out what's going on.
I just had a colonoscopy; there were sporatic tiny white spots all over my colon the the pics that the doc gave me. The spots are normal colonic bacteria.