There was a kitty with neuropathy in PZI a couple of years ago that got B-12 for year before it went away. Point is, it did get better, just took a long time.
Mel's point about potassium is a good one. That was Bob's battle, low potassium. It took probably three months after he was OTJ before his levels got into the normal range. It is something you have to do slowly. But the most important thing is that you have to know what the level is, and it has to be monitored by blood tests. Bob was tested every 3-4 weeks. Because too high is just as risky as too low. Potassium affects all muscles and the most important muscle they have is their heart. If the costs are prohibitive, I'd stay with the B-12 and at some point in the future when a blood panel is an option, and if he still shows muscle weakness, try to have one done. But give the B-12 a chance as it can take quite a while to work it's magic.