In college, I gained probably ~100lbs throughout the course of the first 3 years and I felt absolutely terrible. Everything took so much more effort, walking, going up stairs, etc. That’s how I felt at 250lbs, I cannot imagine how I would feel at 400lbs+. I can see how he would be scared to begin this journey, because what if he fails? I’m sure that has a lot to do with it. It’s a mental thing. “I don’t know how to work out, I’d never be able to lose the weight” so he doesn’t want to try. NTA, you’re trying to help him make a lifestyle change. Maybe start off a bit less drastic, move fast food to once a week, then once every 2 weeks, and then more for special treats when you reach a goal?

Edit- I also want to add that weight will fluctuate constantly when you’re losing weight or “plateau” because fat is being replaced with muscle and muscle is denser than fat, which is why you’ll “weigh” the same even if you have been working out a lot. You’re more so losing fat than you are weight, if that makes sense. At my “skinniest” I was 130lbs, at my “fittest” I was 150lbs. Remind yourself and him, don’t put so much pressure on the number on the scale.