
sorry to hear about the rta, hope everyone involved is ok! perhaps you could ask a few friends and family to come round so he still gets to meet people of different ages, sex and appearance, oerhaps they could bring some well behaved adult dogs as well, so he at least gets to see them and perhaps interact with them to a point.
depending how severe his injuries are you could perhaps carry him out into your street several times a day, and get him used to seeing traffic and crowsds etc (depending on where you live)
wih regards to the car,start by taking him in and not starting engine, perhaps sitting with him in boot and give him a tasty treat. then proceed to starting engine, and eventually do some short 5minute journeys, all positive and at his own pace.