This might be a gastroenteritis- virus being the most common cause.
There are no magical remedies that will stop his stools immediately.
Be sure he drinks well.
You might give him plenty of water, ORS
, vegetable soups, Rice based Porridge, Boiled potato, Banana and plenty of curd.
Avoid cow`s milk and fruit juices for few days as it might cause increase in frequency of stools. You may give him soya milk instead.
You may give him probiotic
sachets- like Econorm- 1/2 sachet twice a day for 5 days.
If he has vomiting
too, you may give him syrup Domperidone
according to his weight.
Be sure he passes urine at least 4 to 5 times in a day.
If he is not drinking anything/ urine frequency is very less/ he is dull and tired/ you see blood in his stools/ he develops high grade fever- do not hesitate to visit the ER.
Hope my answer helps you.
Wish your son a speedy recovery.