Give me a month or so with him.
There would be no junk food in the house for him to have access to.
Meat, starch & a vegetable. Cereal or eggs sausage & toast in the morning. I won't let or make him starve, because I would not do that to him. Plates of food would be given to him. He will HAVE to eat it eventually. No junk food. Only once every blue moon.
He needs to be shown some tough love. And it would take that kind of love to get him back on track. But it would be his parents' job to make sure that he STAYS on track. Otherwise he'll backslide and go back to his old years-long junk food eating days!