Hi Edith, I'm trying to sort this out, if you don't mind me thinking out loud.
OK, the kitten has no tail and the vet says this is a genetic defect, but the kitten is using the litter box on his own. Well, I agree with Critters, having no tail sounds like Manx Syndrome. I will trust your vet, if it was a fan belt injury s/he would have been able to still see the injury since the kitten is only 6-7 weeks old. So let's assume he was born without a tail. Manx cats can have anywhere from normal ability to eliminate to being unable to eliminate without help. One side effect of Manx Syndrome can be megacolon. At the end of this message I am putting a link to information on megacolon. Hopefully this will not be an issue with this kitten, since he appears to be eliminating normally.
However, this is where I get lost. As far as I know, when Manx Syndrome affects the hind end, you may have a kitten who hops, but I don't remember anything about Manx kittens dragging. Therefore, I am thinking you have two things going on. You have a kitten born with Manx syndrome, and now the little guy has an injury from being thrown from the car. In spite of what they say about cats always landing on their feet, this does not apply to a SEVEN WEEK OLD KITTEN THROWN FROM A MOVING CAR.
Therefore, I think I would treat this as a back injury and crate this kitten and not let him drag around, and see if his mobility improves. (As a complete side issue, I will put a picture below showing a way to support a kitten to help him walk, so you can improvise something, but I feel that will be for later ***after*** the crate rest.)
I'm not sure what all you had to do with your wheelchair dog, whether it was an injury or simply old age and hip dysplasia. If you have experience with a back injury then your know about crate rest. You simply put the little guy in a small crate (one of those rabbit-sized wire pens would work) with everything he needs to make him comfortable, and force him to rest for several weeks. A low sided litter box like a jelly roll pan might be easiest for him. If the back is injured, this will give it a chance for the swelling to go down and hopefully his walking will improve. If your vet agrees about the possbility of back injury, s/he may prescribe steroids in addition to the crate rest, either oral or at least an injection.
Several people here have had cats who were dragging regain mobility through crate rest, including Bendy and Debbie. Hopefully one of them will be along soon.
As for the diarrhea, I think I would ask about the normal kitten worming, whatever is appropriate for that age, and see if it helps.
The picture below is from http://www.caringcanine.com/clients.htm CLICK HERE info on megacolon in Manx cats