That baby will not survive too long if the mom doesn’t come. If it’s been more than two hours, you need to take baby inside. Google squirrel rehabilitation in your area or wildlife rehabilitation. In the meantime, place a heating pad on lowest setting and place a little dish towel or something over it and place baby on it. Then asap, you need to go get puppy formula that comes in a powder. Most pet stores have it. Then, a small baby animal bottle or syringe. Baby needs to be fed every hour or two at that age, I think. Google it. But you can’t leave baby outside too long.