You must keep that baby warm! If it's cold, it will die.
Go to your local pet store and tell them you have a baby squirrel. They will give you a can of formula that will replace mother's milk. Do NOT feed him cow milk!!!!! It can kill him! They will also give you a syringe to use for feeding him. After that, it's just like a human baby: feed it every few hours, as much as he wants, (not more, though). If he lives through the night, then you've got a winner. See if you can release him into the wild when he's a bit bigger, (no longer on milk and learning to eat nuts and sweet fruit...if you're going to release him, make sure that you give him some nuts that are only partially cracked so he learns that he must get into the nut before he can eat it).
Flying squirrels make great pets, and domesticate easily, but the larger non-flying squirrels do NOT. See if you can get in touch with your local wildlife rehab. Good luck!
NOTE: Once he's warm and fed, he'll stop squeeking until he's hungry again.