I'm not a lawyer and you know that of course...
Part of me wants to say I wouldn't even bother with his insurance company anymore. Sue HIM - he hit you and it is up to HIM to make it right. He is supposed to have insurance for this but if his insurance is not fulfilling his obligation based on the determination of fault then the responsibility is his. Ultimately his insurance is a service for him paid for by him, the state requires us to have it to protect both ourselves and insure that the other guy (you) has some coverage.
Let say his insurance would pay but the coverage he had did not cover the entire amount of your bill. He would be liable for the rest I believe. Lets say you have a 50k car but his liability insurance only covered 30k (I think the minimum required by law is much less).
Again, not a lawyer.
I have used this site now and again to ask basic legal questions.
http://www.lawguru.com/
I'm just sayin.