dear user first i would like to tell you that diarrhea is very common problem in infants and you need not to worry about that but you should do simple steps....... allergy to certain milk formula that baby take and then you discuss with your doctor to change milk formula and if not so his milk can be given at any time diarrhea may have more than one cause but the most common cause in that age 9 month is viral infection of the intestine ....viral infection causes severe watery diarrhea and may lead to complication as dehydration...... you should give your grandson oral rehydration therapy and fluids...... oral rehydration sachets administered to protect the baby from dehydration and it is the most important step in management...if you do that your granson will be fine shortly and improve..... so milk can be taken with no problems with your garandson any time if he has no milk allergy ....but oral rehydration fluids will help him to improve hope that helps