What Causes Soft, Bad Smelling Stool After Gastritis?

My 5 year old recently had some gastritis where he had vomiting, no fever . Lasted off & on for 3 days. Now he is back on milk and a more or less regular diet, but has soft foul smelling yellow stools (not liquid.) Will these cease soon, I have been told to let something like this work itself out. He has a good appetite and no further symptoms.