It's good advice not to panic - some dogs have tender digestive systems. But it could also be your dog's food is not agreeing with him or it has something he's allergic to.
Lamb and rice foods can be gentle on the stomach and are often suggested for allergic dogs.
Also - be sure your dog isn't getting into foods that could cause problems. See the following article for more information:
Dangerous Foods - Surprising foods that can harm your dog