another thought:

when cats are dying, they seem to know and tend to hide in places they can't be found. If this cat went home, perhaps he didn't feel like he was dying?

I'm sure if he had internal injuries and they were bad, you would have heard something by now. I have a feeling he'll pull through!

I forgot, but my parents also had a cat that was hit by a car. My mother found this poor little stray cat in a ditch, struggling to move but his hind end wasn't working. I believe he may have had his pelvis fractured, and he was in the hospital for quite a while. He never walked normally, but he lived for about 10 years after that, and my parents just adored him.