So very sorry to hear about your pup.
If you know who hit him I would contact the proper authorities.
Please don't take this harshly, but it is our job to protect our dogs, even if they want something it is up to us to decide if it is safe for them. He may want to run the streets following your family but it is not safe for him. I'm not say it is your fault but he is really lucky he was not killed.
Perhaps setting up a run? My BF has a doggie door leading out to a X pen and kennel that way his pups can run around, but if they dig they will only dig out into the fenced yard. If it is still a problem perhaps looking into An electronic fence. My dad has a german short hair pointer, he has 2 1/2 acres but loved to get out. My dad trained him with an electronic fence and if he gets close to the edge it beeps, if he gets closer a vibration, then when he starts to cross it shocks, He only got shocked once and he understood and now he knows where to stay. Many say its mean, but it is better than seeing your pet die or injured.
Good luck, and I hope has a quick recovery.