Wow, that poor little guy! Maybe put a blanket around the tub or cage, with the heating pad stuck to the side for warmth. (So he can move away if he gets too hot) Make sure he stays hydrated, do you have an eye dropper to help him get liquids? And just keep an eye on him. They are fragile when sick, so giving them low stress, warmth, food, liquids and the anti-biotics I think is the best you can do. I hope he pulls through for you!
(I'm just applying rat care to this case)