Thank you for your question. As you are aware, it is very important for diabetic animals to eat properly at the time of their injection. This allows them to absorb the energy from their food using the insulin that you have given them in the syringe.
Unfortunately diabetic cats, just like humans, are more prone to infection. It is possible that Ging has an infected tooth that is causing him pain and preventing him from chewing his food. It is also possible that Ging has an oral ulcer. This could be related to kidney disease or an infection.
As diabetic cats can become sick quite quickly, I would strongly recommend visiting Ging’s vet. They will be able to examine his mouth and may wish to perform some blood tests to ensure that Ging does not have any other organ dysfunctions which may be contributing to his sore mouth.