It's great that you're staying with your grandfather to help him with his diabetes!
I did some research and some meters give a reading of "Hi" when blood glucose levels exceed the highest number that the system can handle.
The best way to keep blood sugar levels down (when medication is not working) is regular exercise and a healthy diet. Exercise doesn't have to be strenuous, but if you could walk with your grandfather for about thrity minutes a day it would help tremendously!
For tips on diet advice check out our diabetes diet page.