Do you home test his glucose levels? That would be the first thing I'd check to make sure he isn't very high or very low.
Second to that, it could be neuropathy. From what I understand, this happens when a kitty has uncontrolled sugars for a long time. The nerves get damaged, and it seems to hit the back legs. My kitty had it bad when he was first diagnosed. The good news is once his sugars were controlled, the nerves began to heal and after some time, Blu had a full recovery. Some signs of this are what you are describing as it is uncomfortable to walk, and also walking on the hocks. Instead of walking on the toes, the kitty will walk on his whole "foot"...like from his toes up to the first joint on his back legs. I didn't notice until the vet showed me....do you see something like that?