I have a friend, who is a nurse at my pediatrician's office, whose baby had the same problem, and eventually outgrew it. From conversations surrounding that experience and from reading, I have become familiar with a syndrome known as 'chronic nonspecific diarrhea'. A child with this syndrome is not sick. They eat well, are growing normally and don't seem to have any food intolerances. It is not health threatening like acute diarrhea because it is not associated with any other dehydrating factors such as fever and excessive sweating, and because they can drink when thirsty and keep the fluid down.
Chronic diarrhea is one of the most frequent gastrointestinal complaints brought to pediatricians.
It is unknown what causes it. Gradual but complete recovery is expected by 3 to 4 years. In the meantime, there are some things you can do to help manage it. Be sure there is plenty of fat in his diet. It may help by slowing down the activity of the stomach and intestine.