We didn't see his mother come around. No nest to be found. No siblings either. He was cold and dehydrated.
He curled up in my wife's hand to stay warm, probably missing his litter mates. We kept him warm with an electric blanket, a big sock, and a bunch of leaves and twigs that he could use to build a nest.
Now he's with other squirrels at the The Wildlife Rehabilitation Center of Minnesota. Even though you know he's better off this way, he left a big hole when he left; we found ourselves actually missing him.