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A lot of times when cats are refusing to jump and are holding their tails down it could be lower back pain. I suppose if he had a kidney infection but usually they are acting more sick than you describe....The problem is, he is an older cat so for me to make some recommendations about giving him pain medication you can find over the counter could cause him harm if he has early kidney disease or liver problems. One thing I could have you safely try is to start giving a glucosamine and chondroitin supplement. 250mg per day would be find. You can find this at many health food stores.
If this isn't working you might need to travel to a vet to have a blood test to check organ function to see if he can handle some medications and a physical to make sure lower back pain is truly the problem.