Hi,
my mums cat was also hit by a car about a year ago which resulted in him having a hip replacement. Anyway after a couple of months bed rest he was finally allowed back outside but he was extremely scared. All we could do was to try and coax him to go out and do things such as sit in the back garden with him and walk him around the garden in our arms (bless him). He always was a bit of a wimp before the accident but since the accident he has got worse although he will go outside now. However, he never really goes that far (as soon as you call him he appears) and like moonshadows cat he doesn't stay out for aslong as he use to.
Hope your cat plucks up the courage soon and all you can do is to encourage him as gently as possible.