Thanks for your question.
It is good to know that your son is recovering very well after hospital discharge.
As the symptoms have improved, regular formula milk can be tried now.
Please continue other medications as advised by your doctor and see him as previously arranged.
Occasionally, there could be transient intolerance to Lactose (type of sugar in formula milk) resulting in loose stools, during recovery phase of gastroenteritis
. This happens rarely though and can be treated by stopping/changing to lactose-free milk for few weeks.
I hope I've been able to resolve your query.
Dr Tejas Shah