Hopefully he'll survive the night and you can find a sanctuary for him to be taken to. It's heartbreaking, no doubt, but you can't save them all. It's that cliche "circle of life", lol. I'm not sure how bats do it, but I know mother birds often push sick/weak babies out of the nest because the mother instinctively knows that one will not survive anyway. Perhaps bats do something similar and the mother of this one was not willing to care for it? Who knows.
Good luck!