The other half's cat has been hit by 2 cars now, and he's bounced back. First time into the path of another car, but hey. I think he's learned his lesson now.
First time his leg was dislocated, and had to be reset. It kept popping out, so they shaved the ball off the end of the leg and set it against the pelvis, that put him out for a good few weeks and like yours he had trouble passing water. As he did second time around, in fact he passed blood eventually due to the bruising and the swelling.
They are extremely resilient, and his age will weigh in his favour. I'd give him another 24-48 hours before doing anything else, keep him dosed up and comfortable and leave him be just checking in every now and then.
Thoughts are with you all mate, hoping for the best.