Research fasting and low carb/high fat diets. Start here: https://idmprogram.com/blog/
Cancer cells need carbs to feed/grow. Low carb diets and extended fasts starve the cancer cells naturally. Fasting longer than 24 hours triggers autophagy, which is the body's way of repairing/replacing damaged or old cell components. See here: https://idmprogram.com/fasting-and-autophagy-fasting-25/
Definitely cut sugar completely out of his diet. Sugar spurs cancer growth.