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Hello, this is Dr. Jess. I am sorry to hear about your kitty! unfortunately, there is not a safe and fast acting over the counter pain medication for cats. Cats are particularly sensitive to aspirin and their bodies do not process it as ours do and so it can be dangerous for them to take: http://www.petmd.com/cat/conditions/digestive/c_ct_aspirin_tox.
You can try putting your kitty on a glucosamine chondroitin joint supplement such as Cosequin for cats: http://www.amazon.com/Nutramax-Cosequin-Capsules-80-Count/dp/B000F4W02M but it can take a month of daily administration to see if it is going to be enough to help your kitty. If you feel he is in enough pain to require more immediate relief, then you will need to discuss a prescription pain medication with your vet.