Looks like a medium size rodent, not a mouse. Most pet stores carry a milk replacer. Just born, and Esbilac come to mind. I know some of the local stores carry little emergency kits that have a pack of powdered milk, bottle, spare nipples, and a cleaner.

The warm water bottles is the best idea for something that size.

Best of luck with him! Some wildlife rescues will take in and care for small babies.