OP, I can appreciate how concerning this must be for you.
He has ADHD, and is using food in some way to get his dopamine, and also, seems to have an emotional need for food to "support" him, as he is becoming more reliant on food, he is becoming physically unhealthy, and you're concerned about implications of his long term health.
I was in a pretty similar situation at his age, and I spent years- infact all of my 20s i was on and off diets and begging the GP to help me. By 2020 I had ballooned up to 23stone 10lb, and by that point, honestly the only thing I enjoyed about life was food.
Now, at 34, im in the process of undoing it all.
In 2020 I had a gastric sleeve (which I don't reccommend for your son! If food is an emotional crutch, then the risks of seriously hurting himself are huge- by changing the addiction to alcohol or something else! Or being sick from continuing to over eat runs risks of sleeve rupture and oesophagus tears)
However, what has been useful for me, is starting therapy for binge eating disorder through the eating disorder service which has helped me to undo a lot of the "diet" mindset that was reinforcing the cycle of binge, restrict, binge, binge. Hate myself, binge some more!
And also GLP medication. It turns off food noise, and allows the space to make healthier decisions and I'm not hungry like a bear whilst on it! The only one thing I will say, is that he needs to try and unpick his relationship with food at the same time. It is also a drug many will need to use for the rest of their lives to keep on top of their weight.
If he can start to do some exercise, even getting him to walk half an hour each day is a great start.
He could break it up into 2/3 walks if he has to. Water exercise is great- swimming, walking in water, and aquasize are all really good options whilst he focuses on building his health.
He may also benefit from getting a smart watch, just so he can see his step count, and can work on improving it gradually.
As he loses weight, please try and get him to enjoy things that he can't currently enjoy. It really helps to move life on from his worst.
My journey has been a bit up and down, I went from 23stone 10, to 13stone 9. I then put on about 5 stone, but restarted on GLP 1s, exercising when I can and making better choices. I'm currently 15stone 7. I've come to accept food will never be "easy" for me to navigate, and I need to have a lot of coping mechanisms in place to stop myself from falling back into using food as a comfort.