What Causes Chronic Loose Stool?

My husband always has watery stools. We have been married for 3 years, and I didn't find out about this until we moved in together. The best way to describe it, is when he has a bowel movement, the stool pretty much projectiles out of his rectum, loudly. Sorry, there no way to make that sound nice. What could be causing this? He said it doesn't hurt, or bother him, but I think he's embarrassed to discuss it with me, so I leave it be.