Have you tried reducing the carbohydrates in his diet?
The low carb high fat diet is very popular amongst members of this forum, and a number of type 2 diabetics have posted that they have reduced their medication as a result, and I'm sure I've seen someone say they are now off insulin because of it.
There is masses of info in the low carb section of this site, including ideas for recipes, menus and experiences. Please don't be put off by the 'high fat' part - you get to choose 'healthy' fats, if you
like, and there is a lot of medical evidence recently published about how fat is not nearly as bad for you (and your heart) as we have been told.
If you want links to that evidence, lots of us can provide them.