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Hello. Thanks for the question.
Is he still urinating normally?
How long has he been walking stiff?
Is it just the rear legs or all 4 legs?
Ok thank you.
It sounds like he may have something called Calicivirus. This is a common virus that cats can get. It causes inflammation and pain in the joints. They'll often times be slow to walk, stiff and they will walk a few steps then crouch down and repeat this behavior. It basically causes an infectious form of arthritis.
The treatment is supportive, with pain meds/ anti-inflammatory meds. I usually go with Onsior or Metacam. Even if you do nothing, it'll work itself out over a week or two. If it is not resolving, then the meds help to get them over this.
I hope this helps, let me know if you have any other questions.