He is suffering from gastroenteritis. It is a serious disease. Main danger is from dehydration, for which he may need IV fluids.
So if child is lethargic, unable to drink, not passing urine, tongue dry, then immediately go to doctor for intravenous drip.
Continue breastfeeding.
Give Zinc syrup 20 mg daily for 14 days
If dehydration not severe, give ORS solution as much as child can take
Give khichri, ripe banana, lassi
Other investigation is Urea and
electrolytes
If vomiting still present, give small and frequent feeds.