Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Rio is not feeling himself today, hiding and moaning in discomfort, and growling when his back leg and paw are touched.
Is he bearing weight on his leg?
Does his leg have any obvious swelling or changes in angulation compared to his other rear leg?
Is he eating and drinking if offered?
If he is able to bear some weight on his rear leg and there aren't any noticeable swollen areas or changes in angulation then you can try to confine him closely tonight and see how he comes along overnight. The most common cause of his symptoms are a bite or scratch wound from another kitty or wild animal. While this is very painful and scary for him a difference of 8-12 hours won't make much difference in treatment if he is weight bearing. I hope he is current on his vaccines however, particularly his Rabies vaccine.
You do want to confine him with water, and a litter box. Make sure he is urinating. I would with-hold food overnight incase they need to sedate him to fully examine him and take radiographs or clean a wound.
Then I do recommend having him examined in the morning.
They can examine him, prescribe pain medication and antibiotics.
Please let me know if you have any further questions.