If he has seen 20 doctors and no one found a problem then it’s FOOD! Overeaters are notorious for not be completely honest about the calories they consume, so that IS the problem. Sounds like someone is not being honest in this case. My wife is a RD/RN and she sees this daily. Patients come in and NO WAY are they overeating...or so they always say. Then looking at the diet she find them consuming 5000-10000 calories a day and most from processed sugar. And, one of the biggest issues is that most consume massive quantities of soft drinks like Coke and you’ll never convince them that is big reason for weight gain. Do a little detective work to see what the real issue is and you’ll find it is overeating and drinking sugar drinks.
I own a grocery store. Its the food. 90% of it sucks in multiple ways.