I'd suggest asking your vet again for some pain meds. NSAIDS (meloxicam, rimadyl, etc) are potentially damaging to the kidney (though in a trauma case as this is, not intrinsic kidney disease, I wouldn't be quite so cautious), but narcotics are primarily metabolized in the liver. He should be able to get codeine, fentanyl, etc without damage to he kidney. People have kidney transplants (and just have the one new kidney), and they get plenty of narcotics for the pain.

Good luck, and hope he feels better soon.

Jake's mom