I haven't researched antibiotics enough as @casportpony has to know which one would be best for this situation, hopefully, she will give her input soon. Since he is not eating it may be a good idea to start weighing him on a food scale, so you can track his weight daily.
I'm not sure what you're keeping him in now, but he needs to be in a warm/dry/quiet area inside the house, on soft bedding, and of course, keep food/water in there with him.
I'm not sure what you're keeping him in now, but he needs to be in a warm/dry/quiet area inside the house, on soft bedding, and of course, keep food/water in there with him.