The way I look at it is, you took responsibility for the animal when you made it a pet. As a wild animal, it would be dead now. You pay for a vet to keep him healthy, pay for good food so he doesnt get sick, get him vaccinated against diseases .......... this is just another extension of that responsibility. He wont get sick or feel any bad effects from diabetes unless you dont give him his shots.
Of course, to be brutally honest here, he probably wont live another 2 years. 3 tops. He's going to get more and more expensive to care for, have more "accidents", and get moodier as he gets older. He'll never feel better than he does right now, and many days he'll feel worse. He'll probably take that out on you. I dont know how close your family is to the pet, but .......... have you ever watched anything or anyone you cared about die slowly? You will, and its not fun. When the time gets close, I would seriously consider euthanizing him, it would be alot more humane.