I dont want to alarm you but i would get him checked - being lethargic and disinterested in play etc are signs of possible dehydtration... when N had gastro Dr told me to look for at least a damp nappy every 6 hrs and to go to hosp if this did not occur...
i would get him checked if it were me, rather be safe and they can dehydrate quickly...
Also you can try pedialyte or similar to replace fluids...
hope he is ok, let us know!