stands for oral rehydration solution. It is powder to be dissolved in measured quantity of water and given to child as often as he drinks. The purpose is to give him adequate fluids and salt during diarrhoeal illness.
Most common cause is virus and may take up to 7 days to recover.
I don't think it is necessary to stop cow milk at this stage. If his diarrhoea gets too prolonged (more than 2-3 weeks) and doesn't show any signs of improvement and his stool examination shows that he cannot digest properly, then only cow's milk may need stopping temporarily.
Dr Tejas Shah