he should pay for the rental car.
he hit your car.
you should not have to be inconvenienced because of his mistake - thus, a rental car is needed.
normally, the insurance will pay for a rental car too.
my car was recently rear ended and the lady's insurance payed the $1700 damage for a whole new bumper PLUS the rental car for two days.
a pain in the ass, but his/her insurance will cover it if they choose.
otherwise, get them to pay out of pocket