15 month old with diarrhea?????

Hi all.

My 15 month old just got diarrhea out of no where.. he pooped twice so far and both times it was diarrhea.. this is the first time this has happened so I’m not sure what to do?? I’m a FTM so idk if I need to take him to ER? Or have him drink more water?? Please any advice would help..

He doesn’t have a fever and is acting normal otherwise.. also is it ok to give him whole milk while he has diarrhea??? TIA