I assume that he is on some sort of diabetic treatment, Insulin or Metformin.
As his blood sugar rises, he will pee a lot and complain of thirst and if the rise in blood sugar and or the symptoms are recognized and dealt with, he will develop a condition called keto-acidosis where he will start to breakdown his body far and protein.
Keto-acidosis is life threatening and is heralded by the following:
Shortness of breath
Breath that smells fruity
Nausea and vomiting
Unconsciousness will follow which at 87 yrs of age may well be fatal.
He is not having enough insulin, make sure he is not having any liquids with sugar in as this will raise the blood sugars quickly as will any foods high in carbs. I would ask the diabetic nurse to come see him.
Have you any fast acting insulin such as Novorapid? A small dose of that i.e. unit of novorapid should bring his blood sugar level down by at least 2 - so say he had 4 units of the insulin his BM should come down by 8.
However, if he is not eating that's a major worry and you should be in touch with his medical team, try and get some fluids into him anyway. He is in danger of ketoacidosis and he needs medical attention. You could phone the hospital where he is usually treated and ask to speak to the Diabetic registrar who is on call for advice, well that is what we do for my daughter who has been a diabetic since she was 2. Please post a message to let us know how he is going on
Having a bit of bother with Bills sugar levels tbh, type 2, they are way too high but at least he's younger than your dear old Grandad, I bet you were worried to bits about him, anyway best news you could have wished for :-)