Newborn Kitten...Help Please.?

Hello, a stray cat abandoned a newborn kitten in my backyard 2 days ago. The kitten is very, very small. When I took him in he still had dry placenta on his fur, and his umbilical cord was fresh. A month ago a kitten was also abandoned in my backyard, and I nursed and raised that kitten and he is healthy and beautiful. However, I have not had as good of luck with this one. It seems he lack a lot of heat although there are plenty of soft t-shirts and towels in his box. I have even put a warm water bottle, yet he has not improved. His condition is very bad, he is lethargic and has had diarrhea. I tried giving him water but he does not swallow and his mouth stays open. I am trying to keep him warm. Please give me tips and advice (besides to take him to the vet because that is not an option right now). What should I do in this moment?