If he is still eating (although) less and is not having diarrhea
(watery or loose stools more than 5 times per day), then, you can wait for some more days before sending to the doctor.
Meanwhile, give him to drink lots of water and fruit juices, together with oral rehydration salts.
If he will not get improvement and feel again sick with diarrhea and/or vomiting
, ask for medical help
Hope it helped!