you should have got out, had a brief confo with him, as he probably would have just said 'doh, my bad. no big deal' and life goes on. easy to say that in retrospect, but i think youre fine. nobody is going to call the police because someone took off after being the victim of a rear-end bump. if you were at fault and took off, then we have a problem. that's probably why he got out in the first place. he was likely thinking that he needed to apologize, so by leaving, im sure he just thought that you were fine and weren't going to worry about it.
long story short, YOURE GOING TO JAIL MOTHER FUCKER