Alot of teens have very bland eating habits. It does expand as they get older. However it would help perhaps to have a nutritionist give him advice as he wants to train alot

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. 💜