I took my 5 month old to the doctors today. She said he had a virus(cough, cold, being sick and the runs) she said to…

I took my 5 month old to the doctors today. She said he had a virus(cough, cold, being sick and the runs) she said to give him so water if i think he's getting dehydrated. I give him his milk and it just comes straight out either end. How much water should I give him and how often ? Really starting to get worried 😩