Actually people rarely have symptoms with high blood sugar as I know from bitter experience. How high is your dads blood sugar? In 2016 rose to over 30 with no symptoms. The only reason I found out was because I was admitted to hospital for heart problems. My blood glucose was tested because I was diabetic type 2 on metformin only and my doctor had said I did not need to test regularly so I did not. The blood glucose was then tested and it was over 30. However, the doctors did say that was not classed as a medical emergency so 999 would not have been appropriate, though 111 would. 2 weeks later I was seen by the diabetes centre and put on insulin and it has mostly stayed between 4 and 7 depending on what I eat.
I think you need to check with your Dads doctor what would be classed as high and what as dangerously high.