What Can I Do About An Upset Stomach In A Toddler Every Time The Child Consumes Milk And Wheat?

My boy is 16month + whenever he drink cow milk or wheat his stomach get upset and it become too hard nd he start crying for that I gave him D Colic medicine.Now doctor said to stop wheat cow milk nd sugar.nd suggested Zerolac and Easyfood.But my baby don t drink Zerolac but drink Isomil so can I switch.