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.
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.