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I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
It does sound like a virus for sure and some kids just take longer to shake it. The stool specimen is really not helpful since he's not having diarrhea and most of the time is not going to change anything about how you treat a gastroenteritis. The reason for this is that the overwhelming majority are viral and don't show up on the culture and bacterial gastroenteritis is rare and presents with fever, chills, and often bloody diarrhea.
You might pick up one of the over the counter acid medications such as zantac or tagamet as they will sometimes help soothe the stomach since it is likely all he really has is a residual gastritis (irritation of the stomach) due to the virus and it will go away soon I assure you.
You may allow him to eat and drink anything he wants as it will not really affect the course of the illness.