Chronic diarreha is a symptom of a food allergy, typically milk. It is not normal. It is a sign that he is not absorping his nutrients and his digestive system is starting to break down. Did he have chronic ear infections, colicky, seemed to be fatigued, perhaps he has small dark circles under his eyes. All things to denote when looking into food allergies.
Keep in mind that doctors are nortorious for not understanding how nutrition effects our lives, our bodies and our minds...they only get one semester of a nutrition course and it is an elective..scary. You definitely need to see a specialist, Pediatric GI as some suggested.
Two recommended books. Digestive Wellness for Children and Is This My Child by Dr. Doris Rapp www.drrapp.com both talk about food allergies and food intolerances (there is a difference) and how to treat it or do a food elimination diet to figure it out for yourself. (This is good for anyone who cannot or does not have healthcare, but nonetheless, easy to do for anyone)
Your child will need to begin some probiotics to help rebuild the good bacteria in his system (I always recommend FloraBaby by Renew Life, excellent brand, found at www.smartbomb.com) and he will also need some fish oils to help reduce internal inflammation...it is also great for the developing brain. for kids under 2, Nordic Naturals, is a good, clean quality brand. Not all probitoics or fish oils are created equal.
Feel free to email me if you have more questions, you received some great info.
J. W. MPH
Wellness Educator/Lifestyle and Wellness Consultant