Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your fellow came home unable to bear weight on on leg, although he is able to move his leg.
Can you see any swelling on his leg or changes in angulation/conformation of that leg?
Any evidence of external trauma (bites/scrapes/scratches)?
Is he running a fever (rectal temperature greater than 103F)?
Usually with a simple sprain they are able to bear weight on the injured leg, although they may not be able to bear full weight.
In cases where a kitty is unable to bear weight I am concerned that there may be a fracture or a ligament or tendon tear/rupture. It also possible that he has a bad abscess that it making it very painful to put pressure on that leg. Any of those things need to be addressed with a veterinary visit however. These are not things that can be treated at home.
If you cannot have him examined immediately I would confine him to a small area, like a bedroom or bathroom, with food
, water and his litter box so he cannot injure himself further.
Please let me know if you have any further questions.