great of you to be helping him.
High blood sugar can make a person feel really bad, as well as causing longer term complications.
Regarding the tiredness, he should also have his thyroid checked, in case that contributes.
The best way I know to get great blood sugar control is by cutting carbs. This is the now popular "keto" diet, but focusing on eating enough protein and low carb veges, rather than eating too much fat. In some cases this can even reverse T2 / put it into remission.
There are some great online resources, though i usually refer people to low carb sources such as "diet doctor" or Bernstein Diabetes University on utube.
Low carb diet means cutting out all sugars, grains, processed foods and fruits, limiting starchy veges.
It means eating eggs, meats, fish, nuts, full fat dairy (cream, butter, cheese), eating non-starchy veges including all greens, beans, egg plant, okra, tomato, cucumber, etc.
If he cuts his carbs, he may also get to reduce his medications in time.
Hope this is helpful.