Sorry to hear about your dad.
He probably won't want to eat or drink anything other than what he feels like eating and drinking.
You say that his blood sugars are too high, but that he has not been diagnosed with diabetes. Is he testing his blood sugar? Is his high blood sugar a result of his therapy? Sorry to sound so ignorant, I'm trying to understand your dad's situation.
How can you know what to do on your own? Your dad has told you not to seek medical advice, but he need not know if you telephone NHS Direct for guidance. They will be able to assess your dad's current problem and organise the practical help you both need. So please give them a call. You will feel relieved not to be shouldering this on your own.
Best wishes.