this my me a difficult experience, but I'm happy you are trying to help the duck. It seems you have already looked him over, but if you suspect being hit by a car I'd be worried about internal injuries and broken limbs... I'm surprised he wasn't killed immediately if that is the case. Could be animal attack? Is he still not using his legs? I'd really check those out if you can handle him gently to see if the legs are bent. If you find any sores, scratches, or wounds I'd put a small amount of neosporin on it.
Since there probably isn't much you can do immediately on your own, I'd at the least try getting him through the night. Keeping him warm and calm in a comfy, safe environment with access to water and food. I'd try providing a few different foods. I'd even go try flipping a rock outside and see if I could find a few worms as that's what he'd likely encounter as a young wild duckling. Then I'd try leaving him alone for 20 or 30 minutes to give him a chance to pick at the food and water. Other than that, I'm not sure what else to do other than hope he makes it through the night until you can call a wild animal vet in the morning.
I've done similar attempts at rescuing young wildlife, once on a baby squirrel fallen from a tree and another on a baby rabbit my cat attacked and "gifted" to me, and unfortunately both did not make it through the night :/ I hope for the both of you that this situation turns out better. Best of luck, and please let us know how he is doing in the morning
