"(diabetic-friendly, dairy-free, gluten-free meals are hard to come by)."
You mentioned the reason for diabetes but not dairy and gluten free foods. Is he intolerant? If he is you will have to weed through the recipes on the following web sites:
http://www.atkins.com/Homepage.aspx
http://www.diabetes.org/food-and-fitness/?loc=GlobalNavFF
http://tinyurl.com/gluten-dairy-sugar-free
First, run to Walmart, and get a blood glucose meter so he can test daily at home. Without one, you're flying blind. Then until you can read up on low carb food preparation, cut all of the "white foods"; not reduce, cut! That means sugar, pasta, rice, potatoes of any kind, any starchy foods. If your grandfather is ambulatory, even a small walk is beneficial in reducing blood glucose levels. Kepp testing everyday, and with no cheating hopefully he'll see the levels decline. Beside diet, medication may be required to further reduce levels.
The first couple of weeks of a low carb diet may be literally quite painful with hunger pains unlike anything felt before. Additionally, some shakiness may be experienced as the body is use to functioning at super saturated glucose levels. But once he gets through that bumpiness, it pretty clear sailing after that. I wish him well.