Ok, my two cents. My parents let me borrow their car in high school, however I didn't have my own car to use exclusively. I was completely responsible for gas, insurance and would have to help pay for any repairs (however I was lucky in that area since my Dad is a mechanic). I had to work hard to have the privelege of using my parents car, by my actions as well as financially.
Personally, I'd make him start helping. If you don't feel comfortable having him pay the entire amount, how about half of it? He has graduated, doing nothing for the summer but working and I don't think it's out of the question for you to require him to assist with the repair costs. Your supplying the car and insurance. That is a major amount of money right there.
Life changes rules mid stream. Your employer changes rules mid stream. You have the right to as well, especially since you've just paid a large amount of money for repairs. JMHO