12m wants to lose weight doing a “crash diet”, how to help him understand nutrition?

My 12-year-old stepson is concerned with his weight. He would be considered overweight for his age group, but in my opinion, it is not that much nor is that a concern for me or his father. growing up he was very skinny. He just started to fill out more at about 9/10, hasn’t gotten much taller or had any major growth spurts . He is prepubescent, so a lot of things can change once gone through puberty. He has seen a nutritionist and they asked him to keep a journal of what he eats. However i know he neglects to include if he drinks a pop, or has a treat. Which defeats the purpose as is for him to be able to look back and see what he has ate, that he didn’t need or wasn’t a healthy option. Being he is a child, he of course if given the option of macaroni or broccoli , is going to choose the macaroni. I remind him, there may be a healthier choice but he will continue to reach for the unhealthier option. I do not want to overly restricted him as I don’t think that’s healthy either. He needs to understand it is ultimately his choices. When he is at our home we eat little to no processed/boxed foods. Grow/butcher our own meat, grow vegetables in summer to freeze or can. Have fruit/vegetable available anytime for snacks etc. He watched some video online of this jacked guy eating only eggs for a period of time, and increased his muscle mass/lost weight. He doesn’t understand that is an adults body/hormones not to mention this guy is clearly putting a ton of work into his appearance and is working out a lot. His dad says is fine to let him try this only eggs diet, I disagree. He has not given a genuine effort to avoid unhealthy food, I don’t think starting crash dieting at 12 is a good pattern to start. His nutritionist obviously also said not good idea. His dad doesn’t think he will stick with it more than a day or two so doesn’t see the harm. However if he does, is that the habit we want him to learn? Not in my opinion. I have talked with him to learn about eating healthier, or being active and of course so has his nutritionist. I feel his “egg diet” is a cop out to avoid learning and understanding how the food you eat affects your body. Do you think letting him try is fine, as he probably will not stick with it? My concern is if he sees weight lost on the scale that will motivate him to continue, developing poor habits that will continue through his life.