My 18 Month Old Son Has Had Diarrhea For 3

My 18 month old son has had diarrhea for 3 going on 4 days now. Its extremely watery like a brown yellowish color. He is not crying in pain and he gets it with anything he eats. His diet hasn't changed so I don't think it has anything to do with food. How long can this go on for? Should he be seen by his Physician? I am keeping him hydrated as best as possible.