Diarrhea.. HELP

My 10 month old has pooped and pooped all day. He woke up COVERED. It's pure water. He's pooped at least more than 10 times today. He is raw and so pitiful. He isn't teething I'm not sure what it is. My question is when do I start to actually worry and take him to the dr/ER? my fear is dehydration but I can't even tell if he is wetting his diapers bc they're so full of poop. HELP 😭😭😭😭