Aggression issue with dog?

see my friend have a four year old german sheperd/red healer mix and he has benn hit by a car about a month or 2 back. but ever since then we cant get near him he run off in the other direction snarling and has his hackles raised. as i said this wasd like 2 months ago when he got hit! whats wrong? he even chased me 2day! see he acually came up to us 2day, so we petted him, but when we walked away he started chasing us and growling so we need help? any idea whats wrong?