The most important piece of information you need to compile and think about is why previous attempts have failed.

You know the saying "the definition of insanity is to keep doing the same thing, hoping for different outcomes"? It's terribly relevant to weight loss, yet everyone keeps ignoring it.

I'm guessing he's tried counting calories and it left him hangry and tired, which wasn't sustainable. Maybe he tried moving more, but it was painful at his weight and ineffective too (exercise pales in comparison to diet). He's probably completely beat down psychologically from the whole world looking at him like a second class citizen and implying he's weak-willed, sloth-like, and a worse human being than normal weight people.

You combine all that it's not clear cut like "doesn't he want to live longer?". Top it all off with how ineffective common weight loss advice is (count and restrict calories regardless of source, do low intensity cardio, and fight through the hunger), and you get stuck in a viscous cycle.

You need to try different things until one of them strikes a chord with him. There are a host of food quality based, ad libitum diets out there that work for many people (Paleo, Atkins, Ornish, McDougal, etc...) They range the spectrum of carnivore to vegan, so there's something for everyone.

There's also baby steps approaches, like eating only whole foods and then going from there.

Also keep in mind, weight is just a proxy for poor health. Eating healthier can make him healthier even if there isn't dramatic weight loss. Visceral fat is more important as a health marker, so are things like insulin resistance, dyslipidemia, hypertension, etc... By cutting out processed food (namely added sugar and refined carbs) he can see massive improvement in his health markers regardless of weight (although I'm sure it would help with weight too).

The bottom line is that at that age and at this point, it's clear that an all or nothing approach is likely ineffective. Adherence will be king so work with him on a variety of different programs and see what sticks. Also, it really helps if other people embark on this journey with him.

You mentioned your entire family has struggled with weight, so it would be very helpful and supportive for all of your to do this together rather than encouraging him to do it in isolation.