Well, the truth of the matter is that it's fairly unlikely that Wilbur will get to remission at this point. It is not impossible, it's just that there are a lot more roadblocks in the way than there are with a newly-diagnosed cat.
The way to get a cat into remission is to basically get them into normal (or close to normal) numbers most of the time, which gives the pancreas a rest and a chance to heal. With a newly-diagnosed cat, the healing process can be quite fast once you remove the stress of too-high BGs. With a longer-term diabetic like Wilbur, it's a lot harder, and often it just doesn't happen.
The good news is, the goal to keep him as healthy as possible by getting him into good regulation overlaps with the requirements for remission, so the thing to do is just keep doing what you're doing. You just never know, he may surprise you down the road, and in the meantime you are giving him a good, healthy life!