Hi Shelley, welcome to you, Mr. Shelley, and Witness (
such a cool name!

).
It's great that you've already transitioned Witness onto a low carb wet diet and are treating him with a long-acting insulin.
Now, lets see how we can help you. First up, lets get the more straightforward bits out of the way.
The problems Witness is having with leg weakness are due to diabetic neuropathy (high blood glucose (BG) levels affecting the nervous system). You can start treating this straight away by giving Witness a daily vitamin B12
methylcobalamin supplement (dose 3,000mcg per day). If you let us know which country you are in we can recommend where to source a diabetic-friendly product. As Witness' BG regulation improves with his insulin treatment the combination will help his legs to strengthen again. To give you an idea of what is possible, here's Tootsie's story:
Unregulated diabetics can't utilise the nutrients from their food properly, hence the weight loss and also the increased hunger that many feline diabetics exhibit. As Witness' BG levels improve he will be able to benefit properly from the food he is eating.
When a diabetic cat is underweight and in the process of becoming regulated it is recommended to feed it extra to help it maintain the weight it still has. As treatment progresses and regulation improves the kitty should regain weight and the amount fed can gradually be eased back to a maintenance amount thereafter.
When using intermediate-acting insulins such as Vetsulin, the 'large feed before each insulin dose' regimen is necessary because these insulins hit quite hard and fast early in each 12-hour cycle and typically peter out several hours before the next dose is due. Lantus has a gentler onset and is longer-lasting, and that allows for much more flexible feeding schedules, and this is helpful especially when a cat needs to regain weight. It's fine to give additional small meals later in each 12-hour cycle, although it is recommended to give most of the food in the first 6 hours after the Lantus dose was administered, when it's at its most active. The only time when food MUST be withheld is for the two hour period before each preshot BG test to ensure that the reading isn't food-influenced.
I'd suggest touching base with your vet and letting them know that Witness is still losing weight and that you need to feed him more until his regulation improves and he regains weight.
Testing suggestions to follow. Watch this space!
Mogs
.