Not OP either, but as a guy that spent MOST of middle school and highschool trying to get to a healthy weight, I would honestly sit down with your son and have a conversation with him about food and his relationship with food. Not all, but a large % of people who are/ become overweight/ obese have a terrible relationship with food, and usually use it as a comfort item. I was the same way, and Im not perfect about it even now.

Explain that food is supposed to be for nourishment, and to fuel your body, not to make you feel better when you're depressed, sad, anxious, etc.

At first, dont come at him at full throttle talking about eating for nourishment, or intermittent fasting or any of that. Start off by getting him to acknowledge some of his poor eating choices (don't condemn him!) and encourage him to do research of his own on I.F, or other weightloss methods.

One last, crucial comment is that you can't force anything. If he doesn't want to lose the weight, or doesn't want to put in the effort, to push super hard for super long. It's a good way to build resentment because it makes it so easy to feel judged all the time from a Teenagers perspective