I doubt very much if he has a case against you if your fences are in good repair, and there is no prior history of your cattle being on the road on several occasions. I had a heifer killed on the highway about 5 years ago and it totalled the guys car. By the marks on the highway, and how far the heifer was from where the impact was, I have good reason to suspect that this guy was travelling far beyond the speed limit. He started to make some noise about me being responsible for the damage to his car, so I checked with the police. They told me that this was considered a similar situation to hitting wildlife on the highway, as the fence was new, and there was no noticeable place we could see where she had crawled out.
On a lighter note, several years ago a drunk driver( another neighbour who had a love affair with a good bottle) hit one of my neighbours Clydesdale horses. The horse was killed, the car totalled and the drunk driver was taken unconscious by ambulance to the hospital. When he came too, the police wanted a report from him. He said that he was going home on a back road ( to avoid the police) after a " few" drinks with friends, and he suddenly saw two rabbits running down the road in front of his car. He said he was surprised when the rabbits stayed right in front of him, so he thought he could hit them both with his car, so he speeded up. The rabbits he thought he saw were the white socks on the Clyde horse.... and he did get them both with his car!
