How can I treat gastro in 9 month old baby?

Keep hydrated. Continue baby's breast milk or formula, he may tolerate smaller amounts at a time if he is vomiting. Watch for at least 3 wet diapers a day. A cotton ball can be useful if he's having watery stools to distinguish stool from urine. Limit his "P fruits" like peaches, pears and prunes, as these may worsen diarrhea. Studies show the sooner babies get back to regular diet, the sooner it resolves.