Potatoes can be stuffed with calories and double baked, bacon, sour cream, minced chives, butter. It may help to let him know that building a powerful body takes plentiful fuel. Dragsters don’t get good fuel mileage. When my son wanted to muscle up as a teen age boy he grabbed some weights to work with and I gave him the wood pile and a splitting maul. When I played college sports I literally ate 6000 calories a day, you have to feed the beast. Fat is the best energy density per volume a person can eat. I wouldn’t worry about nutrition as much as energy. A daily multivitamin can close the nutrition gap temporarily. Do you think the dietary choices are driven by body image concerns? If so a talk about the unreasonable expectations media and peers have and the disproportionate influence they have on the adolescent psyche might be appropriate. Social media is a new threat to well being you and I didn’t have, remember how difficult it was even with just peers being critical? Imagine having the whole world criticizing you from the screen in the palm of your hand. I hope these thoughts help you and your son. 💜