I would also add anti fungals to the diet like garlic, caprylic acid, oregano oil, apple cider vinegar, grapefruit seed extract, olive leaf extract and rotate them each month. Add lots of fresh greens to the diet and experiment with green 'drinks'. This is good healthy stuff for anyone, not just diabetics. Juice fresh veggies and drink them often.
Chromium picolenate is excellent for diabetics too.
If the docs have given him a 'diet' to follow, I wouldn't! It usually wrong and perpetuates the condition. Once you start gathering information online from natural/alternative health sources, you'll find the truth. Happy searching!