Why does my son everytime he drinks whole milk he poops? his 1yr old and his been drinking whole milk now..but recently like 5days now he poops everytime he drinks it but only when he drinks milk. and when he poops its watery yellow and really bad smell..

Lactose : Lactose intolerance is the lack of an intestinal enzyme called lactase and is the inability to digest lactose - the sugar found in milk and other dairy products. Babies are born with the lactase enzyme and most lactose intolerance does not occur until age 3 or older if at all. Symptoms can be bloating, cramps, diarrhea, nausea and poor growth in children. Some races such as asians and african americans are more susceptable to lactose intolerance than caucasians. Some people will have transient lactose intolerance after a gastrointestinal illness which improves over time. You can try giving him lactaid whole milk (milk with the lactase already broken down) and see if he improves. If he just has trouble digesting lactose, this could help. If this persists, or if he is having any weight loss, blood in his stool, rash, or any other sign of more serious illness, call your child's doctor.

Dr Larabee agreed

Milk diarrhea: is most likely caused by a mild intestinal deficiency of the enzyme to digest lactose, the sugar in milk, although allergy to the milk protein or problem with digesting the fats in whole milk is also possible. Lab testing is difficult, so elimination/rechallenge is the best approach. Try a lactose reduced milk or lactose tablets. If cheese is well tolerated, then allergy and fat digestion rare.

It sounds like an allergic reaction and that you should switch to non-dairy. Please take him to a Pediatrician for a thorough examination and treatment recommendations.