that is some cat I would say with that weight it could be diabetic. I have two cats in at the moment both are being injected twice a day with insulin and both have nerve damage to the spin from undiagnosed diabeties. then again It could also be arthritis especially if they are carrying a bit of weight and old their joints can be damaged and so sometimes they feel it more than other times.
I agree with Savas the only way to be certain is to have some blood tests run as soon as possible.
You should get your cat checked out. He's at an age where certain issues can come up, and this sounds like a symptom for some of them. It's hard to say what exactly is causing it, though. It could be related to a thyroid issue, neurological...best to get a blood workup done and find out the proper course of treatment.