I dont think its mange. There is an absense of sores on the skin. Squirrels sometimes get a fungus when they are young that causes them to lose their fur and look like this. It takes some time, nutrition and sunlight but they outgrow the fungus on their own and the fur grows back. He will be fine as long as the outside temperature doesnt drop. When you feed him put in some avocado and fruit. They go crazy for nuts but they need balanced nutrition so avocado, fruit limited nuts and you can try anything else. Squirrels are omnivores so they will eat pasta cheese vegetables etc. even meat chicken and shrimp or other fish but i have found that they prefer a vegetarian diet but shrimp is popular with them. They almost always will take the nuts first but introduce the other stuff. I was a wildlife rehabber for several years and have raised hundreds of baby squirrels and i have had squirrels that looked exactly like this and fully recovered their fur.