yeah i'd be going after him a million miles an hour. If he's admitted liability, then pester him daily for either payment or his ins company's details.
A formal letter to him giving him 7 days to respond with his intentions would've been the way to go....if you don't get anywhere with him and you still want to go with a letter, you might be able to use my companies letterhead. Official letters tend to speak louder than personal ones.
Alternatively, depending on the exact details of the accident, I'm happy to give him a ring for you and see if I have better luck. Unfortunately he probably sees you as someone he can stuff around and draw the whole process out.......people like that annoy me. If he is not accepting liability, that may explain his lack of cooperation.