A change from formula to whole milk?

my son is one years old started drinking whole milk for 2 weeks already from formula his stool is good but when his about to start he starts crying n pushes n its all the time could it be the milk or something else please help me.....he has no teeth yet so his still eating from jars his is 21 lbs n 4 oz.....the doctor recommend to give him 3 times a day koro syrup n half of prums juice