Is he eating too many donuts? Or maybe just too much in general?
Is he a "closet eater" and just doesn't want to tell you what he eats when you are not with him? Does he taste any of the dishes he cooks as a chef? Does he track his food and exercise daily
There is no way to know for sure what someone eats unless you are with them 24/7. If all the medical test are coming back normal I would take a closer look at his food consumption and daily exercise.
"Just Do It"