I'm a new egg also and really don't know a lot about diseases and such, but I have been having issues with some of my chickens and a rooster also that started out this way, so I thought since no one else has answered you, I would try. Does he have any other symptoms like coughing or sneezing or runny nose? Mine had these problems, like a respiratory infection, so on advise I have been treating him with antibiotic Tetracycline. If he just has diarrhea, seems he could have any number of problems, maybe even an infection of some kind, but that's how my problems started. From reading up on all of this, you have the very first thing you do, right. Isolate him in a warm place and keep an eye on him. If he doesn't want to eat or drink for a long time, you will need to address the dehydration issue and give him some supplement in a syringe (without needle) or a dropper to keep him hydrated. If he gets dehydrated, he will weaken and immune system will not be able to fight off whatever it is. I got a bag of Electrolytes at feed store for $4.00, lasts a long time since you only use very little per gallon. Only mix enough to turn the water the color of Mt. Dew. Hope he eats and drinks on his own. Let us know how he progresses, and keep him warm. So sorry he's sick.