I am sorry to hear you are worried about Cat. My name is Rebecca. I am a veterinarian and will do my best to help.
I agree he does not sound happy if he is limping, and can't jump. That sounds like he is painful somewhere, internally or in his joints. Pain medication may help with that.
Is he eating and drinking still? When a cat is really suffering, they usually stop eating. It is good that his labwork was normal a few months ago, but that may have changed.
Losing weight can be a sign of heart disease, cancer, pancreatitis, kidney disease, and hyperthyroidism. If he were mine, I would do a thorough physical exam, recheck his blood tests, and see if pain medication helps him to move around better. However, at his age, he could have a serious illness such as cancer or heart disease.
Let me know what other questions I can answer.